UNDERWRITING AGREEMENT

Exhibit 1.1

EXECUTION VERSION

ONEOK Partners, L.P.

5,000,000 Common Units

Representing Limited Partner Interests

U NDERWRITING A GREEMENT

June 16, 2009

U NDERWRITING A GREEMENT

June 16, 2009

UBS Securities LLC

Citigroup Global Markets Inc.

Merrill Lynch, Pierce, Fenner & Smith Incorporated

as Managing Underwriters

c/o UBS Securities LLC

299 Park Avenue

New York, New York 10171-0026

Ladies and Gentlemen:

ONEOK Partners, L.P., a Delaware limited partnership (the “ Partnership ”), proposes to issue and sell to the underwriters named in Schedule A annexed hereto (the “ Underwriters ”), for whom you are acting as representatives (the “ Representatives ”), an aggregate of 5,000,000 common units (the “ Firm Units ”), representing limited partner interests in the Partnership (the “ Units ”). In addition, solely for the purpose of covering over-allotments, the Partnership proposes to grant to the Underwriters the option to purchase from the Partnership up to an additional 750,000 Units (the “ Additional Units ”). The Firm Units and the Additional Units are hereinafter collectively sometimes referred to as the “ Offered Units .” The Offered Units are described in the Prospectus which is referred to below. ONEOK Partners GP, L.L.C., a Delaware limited liability company and the general partner of the Partnership, is referred to herein as the “ General Partner ”.

The Partnership has prepared and filed, in accordance with the provisions of the Securities Act of 1933, as amended , and the rules and regulations thereunder (collectively, the “ Act ”), with the Securities and Exchange Commission (the “ Commission ”) a registration statement on Form S-3ASR (File No. 333-137419) under the Act (the “ registration statement ”), including a prospectus, which registration statement incorporates by reference certain documents which the Partnership has filed, or will file, in accordance with the provisions of the Securities Exchange Act of 1934, as amended, and the rules and regulations thereunder (collectively, the “ Exchange Act ”). Such registration statement has become effective under the Act.

Except where the context otherwise requires, “ Registration Statement ,” as used herein, means the registration statement, as amended at the time of such registration statement’s effectiveness for purposes of Section 11 of the Act, as such section applies to the respective Underwriters (the “ Effective Time ”), including (i) all documents filed as a part thereof or incorporated or deemed to be incorporated by reference therein and (ii) any information contained or incorporated by reference in a prospectus filed with the Commission pursuant to Rule 424(b) under the Act, to the extent such information is deemed, pursuant to Rule 430B or Rule 430C under the Act, to be part of the registration statement at the Effective Time.

The Partnership has furnished to you, for use by the Underwriters and by dealers in connection with the offering of the Offered Units, copies of one or more preliminary prospectus supplements, and the documents incorporated by reference therein, relating to the Offered Units. Except where the context otherwise requires, “ Pre-Pricing Prospectus ,” as used

herein, means each such preliminary prospectus supplement, in the form so furnished, including any basic prospectus (whether or not in preliminary form) furnished to you by the Partnership and attached to or used with such preliminary prospectus supplement. Except where the context otherwise requires, “ Basic Prospectus ,” as used herein, means any such basic prospectus and any basic prospectus furnished to you by the Partnership and attached to or used with the Prospectus Supplement (as defined below).

Except where the context otherwise requires, “ Prospectus Supplement ,” as used herein, means the final prospectus supplement, relating to the Offered Units, filed by the Partnership with the Commission pursuant to Rule 424(b) under the Act on or before the second business day after the date hereof (or such earlier time as may be required under the Act), in the form furnished by the Partnership to you for use by the Underwriters and by dealers in connection with the offering of the Offered Units.

Except where the context otherwise requires, “ Prospectus ,” as used herein, means the Prospectus Supplement together with the Basic Prospectus attached to or used with the Prospectus Supplement.

“ Permitted Free Writing Prospectuses ,” as used herein, means the documents listed on Schedule B attached hereto and each “road show” (as defined in Rule 433 under the Act), if any, related to the offering of the Offered Units contemplated hereby that is a “written communication” (as defined in Rule 405 under the Act). The Underwriters have not offered or sold and will not offer or sell, without the Company’s consent, any Offered Units by means of any “free writing prospectus” (as defined in Rule 405 under the Act) that is required to be filed by the Underwriters with the Commission pursuant to Rule 433 under the Act, other than a Permitted Free Writing Prospectus.

“ Disclosure Package ,” as used herein, means any Pre-Pricing Prospectus or Basic Prospectus, in either case together with any combination of one or more of the Permitted Free Writing Prospectuses, if any, and the information included in Schedule C hereto.

Any reference herein to the registration statement, the Registration Statement, any Basic Prospectus, any Pre-Pricing Prospectus, the Prospectus Supplement, the Prospectus or any Permitted Free Writing Prospectus shall be deemed to refer to and include the documents, if any, incorporated by reference, or deemed to be incorporated by reference, therein (the “ Incorporated Documents ”), including, unless the context otherwise requires, the documents, if any, filed as exhibits to such Incorporated Documents. Any reference herein to the terms “ amend ,” “ amendment ” or “ supplement ” with respect to the Registration Statement, any Basic Prospectus, any Pre-Pricing Prospectus, the Prospectus Supplement, the Prospectus or any Permitted Free Writing Prospectus shall be deemed to refer to and include the filing of any document under the Exchange Act on or after the initial effective date of the Registration Statement, or the date of such Basic Prospectus, such Pre-Pricing Prospectus, the Prospectus Supplement, the Prospectus or such Permitted Free Writing Prospectus, as the case may be, and deemed to be incorporated therein by reference.

As used in this Agreement, “ business day ” shall mean a day on which the New York Stock Exchange (the “ NYSE ”) is open for trading. The terms “herein,” “hereof,” “hereto,”

“hereinafter” and similar terms, as used in this Agreement, shall in each case refer to this Agreement as a whole and not to any particular section, paragraph, sentence or other subdivision of this Agreement. The term “or,” as used herein, is not exclusive.

The Partnership and the Underwriters agree as follows:

1. Sale and Purchase . Upon the basis of the representations and warranties and subject to the terms and conditions herein set forth, the Partnership agrees to issue and sell to the respective Underwriters and each of the Underwriters, severally and not jointly, agrees to purchase from the Partnership the number of Firm Units set forth opposite the name of such Underwriter in Schedule A attached hereto, subject to adjustment in accordance with Section 8 hereof, in each case at a purchase price of $43.98 per Unit. The Partnership is advised by you that the Underwriters intend (i) to make a public offering of their respective portions of the Firm Units as soon after the effectiveness of this Agreement as in your judgment is advisable and (ii) initially to offer the Firm Units upon the terms set forth in the Prospectus. You may from time to time increase or decrease the public offering price after the initial public offering to such extent as you may determine.

In addition, the Partnership hereby grants to the several Underwriters the option (the “ Over-Allotment Option ”) to purchase, and upon the basis of the representations and warranties and subject to the terms and conditions herein set forth, the Underwriters shall have the right to purchase, severally and not jointly, from the Partnership, ratably in accordance with the number of Firm Units to be purchased by each of them, all or a portion of the Additional Units as may be necessary to cover over-allotments made in connection with the offering of the Firm Units, at the same purchase price per unit to be paid by the Underwriters to the Partnership for the Firm Units. The Over-Allotment Option may be exercised by the Representatives on behalf of the several Underwriters at any time and from time to time on or before the thirtieth day following the date of the Prospectus Supplement, by written notice to the Partnership. Such notice shall set forth the aggregate number of Additional Units as to which the Over-Allotment Option is being exercised and the date and time when the Additional Units are to be delivered (any such date and time being herein referred to as an “ additional time of purchase ”); provided , however , that no additional time of purchase shall be earlier than the “time of purchase” (as defined below) nor earlier than the second business day after the date on which the Over-Allotment Option shall have been exercised nor later than the tenth business day after the date on which the Over-Allotment Option shall have been exercised. The number of Additional Units to be sold to each Underwriter shall be the number which bears the same proportion to the aggregate number of Additional Units being purchased as the number of Firm Units set forth opposite the name of such Underwriter on Schedule A hereto bears to the total number of Firm Units (subject, in each case, to such adjustment as the Representatives may determine to eliminate fractional common units), subject to adjustment in accordance with Section 8 hereof.

2. Payment and Delivery . Payment of the purchase price for the Firm Units shall be made to the Partnership by Federal Funds wire transfer against delivery of the certificates for the Firm Units to you through the facilities of The Depository Trust Company (“ DTC ”) for the respective accounts of the Underwriters. Such payment and delivery shall be made at 9:00 A.M., New York City time, on June 22, 2009 (unless another time shall be agreed to by you and the Partnership or unless postponed in accordance with the provisions of Section 8 hereof). The time

at which such payment and delivery are to be made is hereinafter sometimes called the “ time of purchase .” Electronic transfer of the Firm Units shall be made to you at the time of purchase in such names and in such denominations as you shall specify.

Payment of the purchase price for the Additional Units shall be made at the additional time of purchase in the same manner and at the same office as the payment for the Firm Units. Electronic transfer of the Additional Units shall be made to you at the additional time of purchase in such names and in such denominations as you shall specify.

Deliveries of the documents described in Section 6 hereof with respect to the purchase of the Offered Units shall be made at the offices of Shearman & Sterling LLP at 599 Lexington Avenue, New York, New York, at 9:00 A.M., New York City time, on the date of the closing of the purchase of the Firm Units or the Additional Units, as the case may be.

3. Representations and Warranties of the Partnership . The Partnership represents and warrants to and agrees with each of the Underwriters that:

(a) The Registration Statement has heretofore become effective under the Act; no stop order of the Commission preventing or suspending the use of any Basic Prospectus, any Pre-Pricing Prospectus, the Prospectus Supplement, the Prospectus or any Permitted Free Writing Prospectus, or the effectiveness of the Registration Statement, has been issued, and no proceedings for such purpose have been instituted or, to the Partnership’s knowledge, are contemplated by the Commission;

(b) the Registration Statement complied when it became effective, complies as of the date hereof and, as amended or supplemented, at the time of purchase, each additional time of purchase, if any, and at all times during which a prospectus is required by the Act to be delivered (whether physically or through compliance with Rule 172 under the Act or any similar rule) in connection with any sale of Offered Units, will comply, in all material respects, with the requirements of the Act; the conditions to the use of Form S-3 in connection with the offering and sale of the Offered Units as contemplated hereby have been satisfied; the Registration Statement constitutes an “automatic shelf registration statement” (as defined in Rule 405 under the Act); the Partnership has not received, from the Commission, a notice, pursuant to Rule 401(g)(2), of objection to the use of the automatic shelf registration statement form; as of the determination date applicable to the Registration Statement (and any amendment thereof) and the offering contemplated hereby, the Partnership is a “well-known seasoned issuer” as defined in Rule 405 under the Act; the Registration Statement meets, and the offering and sale of the Offered Units as contemplated hereby complies with, the requirements of Rule 415 under the Act (including, without limitation, Rule 415(a)(5) under the Act); the Registration Statement did not, as of the Effective Time, contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading; each Pre-Pricing Prospectus complied, at the time it was filed with the Commission, and complies as of the date hereof, in all material respects with the requirements of the Act; at no time during the period that begins on the earlier of the date of such Pre-Pricing Prospectus and the date such Pre-Pricing Prospectus was filed with the Commission and ends at the time of purchase did or will

any Pre-Pricing Prospectus, as then amended or supplemented, include an untrue statement of a material fact or omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading, and at no time during such period did or will any Pre-Pricing Prospectus, as then amended or supplemented, together with any combination of one or more of the then issued Permitted Free Writing Prospectuses, if any, include an untrue statement of a material fact or omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading; each Basic Prospectus complied or will comply, as of its date and the date it was or will be filed with the Commission, complies as of the date hereof (if filed with the Commission on or prior to the date hereof) and, at the time of purchase, each additional time of purchase, if any, and at all times during which a prospectus is required by the Act to be delivered (whether physically or through compliance with Rule 172 under the Act or any similar rule) in connection with any sale of Offered Units, will comply, in all material respects, with the requirements of the Act; each of the Prospectus Supplement and the Prospectus will comply, as of the date that it is filed with the Commission, the date of the Prospectus Supplement, the time of purchase, each additional time of purchase, if any, and at all times during which a prospectus is required by the Act to be delivered (whether physically or through compliance with Rule 172 under the Act or any similar rule) in connection with any sale of Offered Units, in all material respects, with the requirements of the Act (in the case of the Prospectus, including, without limitation, Section 10(a) of the Act); at no time during the period that begins on the earlier of the date of the Prospectus Supplement and the date the Prospectus Supplement is filed with the Commission and ends at the later of the time of purchase, the latest additional time of purchase, if any, and the end of the period during which a prospectus is required by the Act to be delivered (whether physically or through compliance with Rule 172 under the Act or any similar rule) in connection with any sale of Offered Units did or will any Prospectus Supplement or the Prospectus, as then amended or supplemented, include an untrue statement of a material fact or omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading; at no time during the period that begins on the date of such Permitted Free Writing Prospectus and ends at the time of purchase did or will any Permitted Free Writing Prospectus include an untrue statement of a material fact or omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading; provided , however , that the Partnership makes no representation or warranty in this Section 3(b) with respect to any statement contained in the Registration Statement, any Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus in reliance upon and in conformity with information concerning an Underwriter and furnished in writing by or on behalf of such Underwriter through you to the Partnership expressly for use in the Registration Statement, such Pre-Pricing Prospectus, the Prospectus or such Permitted Free Writing Prospectus; each Incorporated Document, at the time such document was filed with the Commission or at the time such document became effective, as applicable, complied, in all material respects, with the requirements of the Exchange Act and did not include an untrue statement of a material fact or omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were

made, not misleading;

(c) prior to the execution of this Agreement, the Partnership has not, directly or indirectly, offered or sold any Offered Units by means of any “prospectus” (within the meaning of the Act) or used any “prospectus” (within the meaning of the Act) in connection with the offer or sale of the Offered Units, in each case other than the Pre-Pricing Prospectuses and the Permitted Free Writing Prospectuses, if any; the Partnership has not, directly or indirectly, prepared, used or referred to any Permitted Free Writing Prospectus except in compliance with Rule 163 or with Rules 164 and 433 under the Act; assuming that such Permitted Free Writing Prospectus is so sent or given after the Registration Statement was filed with the Commission (and after such Permitted Free Writing Prospectus was, if required pursuant to Rule 433(d) under the Act, filed with the Commission), the sending or giving, by any Underwriter, of any Permitted Free Writing Prospectus will satisfy the provisions of Rule 164 and Rule 433 (without reliance on subsections (b), (c) and (d) of Rule 164); the conditions set forth in one or more of subclauses (i) through (iv), inclusive, of Rule 433(b)(1) under the Act are satisfied, and the registration statement relating to the offering of the Offered Units contemplated hereby, as initially filed with the Commission, includes a prospectus that, other than by reason of Rule 433 or Rule 431 under the Act, satisfies the requirements of Section 10 of the Act; neither the Partnership nor the Underwriters are disqualified, by reason of subsection (f) or (g) of Rule 164 under the Act, from using, in connection with the offer and sale of the Offered Units, “free writing prospectuses” (as defined in Rule 405 under the Act) pursuant to Rules 164 and 433 under the Act; the Partnership is not an “ineligible issuer” (as defined in Rule 405 under the Act) as of the eligibility determination date for purposes of Rules 164 and 433 under the Act with respect to the offering of the Offered Units contemplated by the Registration Statement; the parties hereto agree and understand that the content of any and all “road shows” (as defined in Rule 433 under the Act) related to the offering of the Offered Units contemplated hereby is solely the property of the Partnership;

(d) as of the date of this Agreement, the Partnership has an authorized and outstanding capitalization as set forth in the sections of the Registration Statement, the Pre-Pricing Prospectuses and the Prospectus entitled “Capitalization” and “Description of Common Units” (and any similar sections or information, if any, contained in any Permitted Free Writing Prospectus), and, as of the time of purchase and any additional time of purchase, as the case may be, the Partnership shall have an authorized and outstanding capitalization as set forth in the sections of the Registration Statement, the Pre-Pricing Prospectuses and the Prospectus entitled “Capitalization” and “Description of Common Units” (and any similar sections or information, if any, contained in any Permitted Free Writing Prospectus), except for such adjustments as are necessary to reflect the offer and sale of the Offered Units;

(e) as of April 30, 2009, the limited partners of the Partnership held limited partner interests in the Partnership aggregating a 98.0% Partnership Interest (as defined in the Third Amended and Restated Agreement of Limited Partnership of the Partnership, as amended (the “ Partnership Agreement ”)) (subject to the provisions of the Partnership Agreement), such limited partner interests being represented by a total of 54,426,087

outstanding Units and 36,494,126 outstanding Class B Units (collectively, the “ Limited Partner Units ”); as of the time of purchase and each additional time of purchase, if any, the Limited Partner Units and the limited partner interests represented thereby were authorized by the Partnership Agreement and are validly issued, fully paid (to the extent required under the Partnership Agreement) and non-assessable (except as described in the Partnership Agreement and except as set forth in Sections 17-303, 17-607 and 17-804 of the Delaware Revised Uniform Limited Partnership Act (the “ DRULPA ”));

(f) the Partnership has been duly formed and is validly existing under the DRULPA and each of its “significant subsidiaries” (as defined in Rule 1-02 of Regulation S-X) (the “ Subsidiaries ”) has been duly formed, is validly existing as a corporation, limited liability company or limited partnership, as the case may be, and is in good standing under the laws of the jurisdiction in which it is formed. The Partnership has all the requisite partnership power and authority and each of its Subsidiaries has full corporate, limited liability company or partnership power, as the case may be, and authority to own or lease, as the case may be, and to operate its properties and conduct its business in all material respects as described in the Disclosure Package and the Prospectus, and is duly qualified or registered to do business as a foreign entity and is in good standing under the laws of each jurisdiction which requires such qualification or registration, except where the failure to be so qualified or registered or in good standing would not reasonably be expected to (i) have a material adverse effect on the condition (financial or otherwise), earnings, cash flow, business affairs or business prospects of the Partnership and its subsidiaries, considered as one enterprise (a “ Material Adverse Effect ”), or (ii) subject the limited partners of the Partnership to any material liability or disability;

(g) all the outstanding equity interests of each Subsidiary have been duly and validly authorized and issued in accordance with such Subsidiary’s governing documents and are fully paid (in the case of any Subsidiary that is a limited liability company, to the extent required by such Subsidiary’s limited liability company agreement, and in the case of any Subsidiary that is a limited partnership, to the extent required by such Subsidiary’s agreement of limited partnership) and nonassessable (in the case of any Subsidiary that is a limited liability company, except as such nonassessability may be affected by Sections 18-607 and 18-804 of the Delaware Revised Limited Liability Company Act or by Sections 2030, 2031 and 2040 of the Oklahoma Limited Liability Company Act, and in the case of any Subsidiary that is a limited partnership, except as such nonassessability may be affected by Sections 17-303, 17-607 and 17-804 of the DRULPA and matters included in such Subsidiary’s agreement of limited partnership), and all outstanding equity interests of the Subsidiaries are owned by the Partnership either directly or through wholly owned subsidiaries free and clear of any perfected security interest or any other security interests, claims or liens (“ Liens ”), except for any such Liens on the outstanding equity interests of the Subsidiaries that are described in the Disclosure Package and the Prospectus;

(h) the Offered Units have been duly and validly authorized and, when issued and delivered against payment therefor as provided herein, will be duly and validly issued, fully paid (to the extent required under the Partnership Agreement) and non-

assessable (except as described in the Partnership Agreement and except as set forth in Sections 17-303, 17-607 and 17-804 of the DRULPA) and free of statutory and contractual preemptive rights, resale rights, rights of first refusal and similar rights; the Offered Units, when issued and delivered against payment therefor as provided herein, will be free of any restriction upon the voting or transfer thereof pursuant to the Partnership’s Partnership Agreement or any agreement or other instrument to which the Partnership is a party; and the statements under the caption “Description of Common Units” in the Basic Prospectus and contained or incorporated by reference in the Registration Statement, insofar as such statements summarize certain provision of documents referred to therein, fairly summarize such provisions in all material respects;

(i) the Units, including the Offered Units, conform in all material respects to the description thereof, if any, contained or incorporated by reference in the Registration Statement, the Pre-Pricing Prospectuses, the Prospectus and the Permitted Free Writing Prospectuses, if any; and the certificates for the Offered Units are in due and proper form;

(j) this Agreement has been duly authorized, executed and delivered by or on behalf of the Partnership;

(k) neither the Partnership nor any Subsidiary is in violation or default of (i) any provisions of the Partnership Agreement or other governing documents of the Partnership or the governing documents of such Subsidiary, as the case may be, (ii) the terms of any indenture, contract, lease, mortgage, deed of trust, note agreement, loan agreement or other agreement, obligation, condition, covenant or instrument to which it is a party or bound or to which its property is subject, or (iii) any statute, law, rule, regulation, judgment, order or decree of any court, regulatory body, administrative agency, governmental body, arbitrator or other authority having jurisdiction over the Partnership or such Subsidiary or any of its properties, as applicable, except in the case of clauses (ii) and (iii) as would not reasonably be expected to have a Material Adverse Effect or as could not materially impair the ability of the Partnership to perform its obligations under this Agreement;

(l) neither the issuance and sale of the Offered Units nor the consummation of any other of the transactions herein contemplated nor the fulfillment of the terms hereof will conflict with, result in a breach or violation of, or imposition of any lien, charge or encumbrance upon any property or assets of the Partnership or any of its Subsidiaries pursuant to (i) the provisions of the Partnership Agreement or other governing documents of the Partnership or any of the governing documents of any of its Subsidiaries, (ii) the terms of any indenture, contract, lease, mortgage, deed of trust, note agreement, loan agreement or other agreement, obligation, condition, covenant or instrument to which the Partnership or any of its Subsidiaries is a party or bound or to which its or their property is subject, or (iii) any statute, law, rule, regulation, judgment, order or decree applicable to the Partnership or any of its Subsidiaries of any court, regulatory body, administrative agency, governmental body, arbitrator or other authority having jurisdiction over the Partnership or any of its Subsidiaries or any of its or their properties, which conflicts, breaches, violations or defaults, in the case of clauses (ii) or (iii), would reasonably be expected, individually or in the aggregate, to have a Material Adverse Effect;

(m) no consent, approval, authorization, filing with or order of any court or governmental agency or body is required in connection with the transactions contemplated herein except (i) such as have been obtained under the Act, and (ii) such as may be required under the blue sky laws of any jurisdiction or the by-laws and rules of the Financial Industry Regulatory Authority (“ FINRA ”) in connection with the purchase and distribution by the Underwriters of the Offered Units in the manner contemplated herein and in the Disclosure Package and the Prospectus or (iii) such that the failure to obtain would not reasonably be expected to result in a Material Adverse Effect;

(n) except for the obligation of the General Partner under the Partnership Agreement to make such investments as are required in order to maintain a 2% interest in the Partnership, such rights as have been duly waived and as described in the Registration Statement (excluding the exhibits thereto), each Pre-Pricing Prospectus and the Prospectus, (i) no person has the right, contractual or otherwise, to cause the Partnership to issue or sell to it Units or other equity interests of the Partnership, (ii) no person has any preemptive rights, resale rights, rights of first refusal or other rights to purchase Units or other equity interests in the Partnership and (iii) no person has the right to act as an underwriter or as a financial advisor to the Partnership in connection with the offer and sale of the Offered Units; except for such rights as have been duly waived, no person has the right, contractual or otherwise, to cause the Partnership to register under the Act any Units or other equity interests in the Partnership, or to include any such Units or other interests in the Registration Statement or the offering contemplated thereby;

(o) the Partnership and its Subsidiaries possess all licenses, certificates, permits and other authorizations issued by the appropriate federal, state or foreign regulatory authorities necessary to conduct their respective businesses, except for such licenses, certificates, permits and other authorizations that, if not obtained, would not reasonably be expected to have, individually or in the aggregate, a Material Adverse Effect, and neither the Partnership nor any such Subsidiary has received any notice of proceedings relating to the revocation or modification of any such certificate, authorization or permit which, individually or in the aggregate, if the subject of an unfavorable decision, ruling or finding, would reasonably be expected to have a Material Adverse Effect, except as set forth in or contemplated in the Disclosure Package and the Prospectus;

(p) other than as set forth in the Pre-Pricing Prospectus, the Prospectus and the Registration Statement, no action, suit or proceeding by or before any court or governmental agency, authority or body or any arbitrator involving the Partnership or any of its Subsidiaries or its or their property is pending or, to the Partnership’s knowledge, threatened that (i) would reasonably be expected to have a material adverse effect on the performance of this Agreement or the consummation of any of the transactions contemplated hereby or (ii) would reasonably be expected to have a Material Adverse Effect;

(q) KPMG LLP, who has certified certain financial statements of the Partnership and its consolidated subsidiaries and delivered its report with respect to the audited consolidated financial statements and schedules incorporated by reference in the

Registration Statement, the Pre-Pricing Prospectuses and the Prospectus, was an independent registered public accounting firm with respect to the Partnership as required by the Act and the Public Company Accounting Oversight Board (United States) and its applicable published rules and regulations as of the issuance of their audit report dated February 28, 2007;

(r) PricewaterhouseCoopers LLP, who has performed certain procedures with respect to certain financial statements of the Partnership and its consolidated subsidiaries included, or incorporated by reference, in the Registration Statement, the Pre-Pricing Prospectuses and the Prospectus, is an independent registered public accounting firm with respect to the Partnership as required by the Act and the Public Company Accounting Oversight Board (United States) and its applicable published rules and regulations;

(s) the historical consolidated financial statements and schedules of the Partnership and its consolidated subsidiaries included or incorporated by reference in the Pre-Pricing Prospectus, the Prospectus and the Registration Statement present fairly in all material respects the consolidated financial condition, results of operations and cash flows of the Partnership as of the dates and for the periods indicated, comply as to form in all material respects with the applicable accounting requirements of the Act and have been prepared in conformity with generally accepted accounting principles applied on a consistent basis throughout the periods involved (except as otherwise noted therein);

(t) the Partnership has obtained for the benefit of the Underwriters the agreement (a “ Lock-Up Agreement ”), in the form set forth as Exhibit A hereto, of each party named in Exhibit A-1 hereto;

(u) the Partnership is not and, after giving effect to the offering and sale of the Offered Units and the application of the proceeds thereof as described in the Disclosure Package and the Prospectus, will not be an “investment company” as defined in the Investment Company Act of 1940, as amended (the “ Investment Company Act ”);

(v) each of the Partnership and each of its Subsidiaries owns or leases all such properties as are necessary to the conduct of its operations as presently conducted except where the failure to do so does not materially interfere with the ownership, operation or benefits of operation of such businesses or materially increase the cost of operation or ownership of such businesses, provided that (a) with respect to the transmission and gathering pipelines of the Subsidiaries that own such pipelines and right-of-way interests related thereto (the “ Pipeline Properties ”), the foregoing shall only constitute a representation that, such Subsidiaries have sufficient title to enable them to use such Pipeline Properties in their businesses as they have been used in the past and as are proposed to be used in the future and will not materially increase the cost of such use, and (b) with respect to any real property, buildings and equipment held under lease by the Subsidiaries, such real property, buildings and equipment are held by the Subsidiaries under valid, subsisting and enforceable leases with such exceptions as are not material and do not interfere with the use made and proposed to be made of such real property, buildings and equipment for such Subsidiary;

(w) the Partnership and its Subsidiaries own, possess, license or have other rights to use, on reasonable terms, all material patents, patent applications, trade and service marks, trade and service mark registrations, trade names, copyrights, licenses, inventions, trade secrets, technology, know-how and other intellectual property (collectively, the “ Intellectual Property ”) necessary for the conduct of the Partnership’s business as now conducted or as proposed in the Prospectus to be conducted, except for failures of ownership or use that would not reasonably be expected to have a Material Adverse Effect. Additionally, (a) to the Partnership’s knowledge, there are no rights of third parties to any such Intellectual Property; (b) to the Partnership’s knowledge, there is no infringement by third parties of any such Intellectual Property; (c) there is no pending or, to the Partnership’s knowledge, threatened action, suit, proceeding or claim by others challenging the Partnership’s rights in or to any such Intellectual Property; and (d) to the Partnership’s knowledge, there is no pending or threatened action, suit, proceeding or claim by others that the Partnership infringes or otherwise violates any patent, trademark, copyright, trade secret or other proprietary rights of others, in the case of each of clauses (a), (b), (c) and (d) which would be reasonably expected to have a Material Adverse Effect;

(x) no labor problem or dispute with the employees of the Partnership or any of its Subsidiaries exists or, to the Partnership’s knowledge, is threatened or imminent, that would reasonably be expected to have a Material Adverse Effect;

(y) none of the following events has occurred or exists: (i) a failure to fulfill the obligations, if any, under the minimum funding standards of Section 302 of the United States Employee Retirement Income Security Act of 1974, as amended (“ ERISA ”), and the regulations and published interpretations thereunder with respect to a Plan, determined without regard to any waiver of such obligations or extension of any amortization period, that would reasonably be expected to have a Material Adverse Effect; (ii) an audit or investigation by the Internal Revenue Service, the U.S. Department of Labor, the Pension Benefit Guaranty Corporation or any other federal or state governmental agency or any foreign regulatory agency with respect to the employment or compensation of employees by any of the Partnership or any of its Subsidiaries that would reasonably be expected to have a Material Adverse Effect; (iii) any breach of any contractual obligation, or any violation of law or applicable qualification standards, with respect to the employment or compensation of employees by the Partnership or any of its Subsidiaries that would reasonably be expected to have a Material Adverse Effect. None of the following events has occurred or is reasonably likely to occur: (i) a material increase in the aggregate amount of contributions required to be made to all Plans in the current fiscal year of the Partnership and its Subsidiaries compared to the amount of such contributions made in the most recently completed fiscal year of the Partnership and its Subsidiaries; (ii) a material increase in the “accumulated post-retirement benefit obligations” (within the meaning of Statement of Financial Accounting Standards 106) of the Partnership and its Subsidiaries compared to the amount of such obligations in the most recently completed fiscal year of the Partnership and its Subsidiaries; (iii) any event or condition giving rise to a liability under Title IV of ERISA that would reasonably be expected to have a Material Adverse Effect; or (iv) the filing of a claim by one or more employees or former employees of the Partnership or any of its Subsidiaries related to

their employment that would reasonably be expected to have a Material Adverse Effect. For purposes of this paragraph, the term “Plan” means a plan (within the meaning of Section 3(3) of ERISA) subject to Title IV of ERISA with respect to which the Partnership or any of its Subsidiaries may have any liability;

(z) the Partnership and its Subsidiaries (i) are in compliance with any and all applicable federal, state and local laws and regulations relating to the protection of human health and safety, the environment or hazardous or toxic substances or wastes, pollutants or contaminants (“ Environmental Laws ”), (ii) have received and are in compliance with all permits, licenses or other approvals required of them under applicable Environmental Laws to conduct their respective businesses and (iii) have not received notice of any actual or potential liability under any Environmental Law, except where such non-compliance with Environmental Laws, failure to receive or comply with required permits, licenses or other approvals, or liability would not, individually or in the aggregate, reasonably be expected to have a Material Adverse Effect, and except as set forth in or contemplated in the Disclosure Package and the Prospectus. Neither the Partnership nor any of the Subsidiaries has been named as a “potentially responsible party” under the Comprehensive Environmental Response, Compensation, and Liability Act of 1980, as amended, except (y) with respect to any matters that, individually or in the aggregate, would not reasonably be expected to have a Material Adverse Effect or (z) as set forth in or contemplated in the Disclosure Package and the Prospectus;

(aa) in the ordinary course of its business, the Partnership periodically reviews the effect of Environmental Laws on the business, operations and properties of the Partnership and the Subsidiaries, in the course of which it identifies and evaluates associated costs and liabilities (including, without limitation, any capital or operating expenditures required for clean-up, closure of properties or compliance with Environmental Laws, or any permit, license or approval, any related constraints on operating activities and any potential liabilities to third parties). On the basis of such review, the Partnership has reasonably concluded that such associated costs and liabilities would not, individually or in the aggregate, reasonably be expected to have a Material Adverse Effect;

(bb) the Partnership has filed all foreign, federal, state and local tax returns that are required to be filed or has requested extensions thereof and has paid all taxes required to be paid by it and any other assessment, fine or penalty levied against it, to the extent that any of the foregoing is due and payable, except for (i) those failures to file or pay that would not reasonably be expected to have a Material Adverse Effect, (ii) any such tax payment, assessment, fine or penalty that is currently being contested in good faith, or (iii) those failures to file or pay set forth in or contemplated in the Disclosure Package and the Prospectus;

(cc) there are no transfer taxes or other similar fees or charges under Federal law or the laws of any state, or any political subdivision thereof, required to be paid by the Partnership in connection with the execution and delivery of this Agreement or the issuance and sale by the Partnership of the Offered Units;

(dd) the Partnership and the Subsidiaries are insured by insurers that the Partnership has no reason to believe are not of recognized financial responsibility against such losses and risks and in such amounts as the Partnership believes are prudent and customary in the businesses in which they are engaged; all policies of insurance and fidelity or surety bonds insuring the Partnership or any of the Subsidiaries or their respective businesses, assets, employees, officers and directors are in full force and effect, except as would not reasonably be expected to have a Material Adverse Effect; the Partnership and the Subsidiaries are in compliance with the terms of such policies and instruments in all material respects; and there are no claims by the Partnership or any of the Subsidiaries under any such policy or instrument as to which any insurance company is denying liability or defending under a reservation of rights clause, except as would not reasonably be expected to have a Material Adverse Effect; neither the Partnership nor any such Subsidiary has been refused any material insurance coverage sought or applied for since December 31, 2007; and neither the Partnership nor any such Subsidiary has any reason to believe that it will not be able to renew its existing insurance coverage as and when such coverage expires or to obtain similar coverage from similar insurers as may be necessary to continue its business at a cost that would not reasonably be expected to have a Material Adverse Effect;

(ee) the Partnership and each of its subsidiaries maintain a system of internal accounting controls sufficient to provide reasonable assurance that (i) transactions are executed in accordance with management’s general or specific authorizations; (ii) transactions are recorded as necessary to permit preparation of financial statements in conformity with generally accepted accounting principles and to maintain asset accountability; (iii) access to assets is permitted only in accordance with management’s general or specific authorization; and (iv) the recorded accountability for assets is compared with the existing assets at reasonable intervals and appropriate action is taken with respect to any differences. Since the date of the most recent audited balance sheet of the Partnership and its consolidated subsidiaries audited by PricewaterhouseCoopers LLP and reviewed by the board of directors of the General Partner of the Partnership, (i) the Partnership has not been advised of (A) any significant deficiencies in the design or operations of internal control over financial reporting that could adversely affect the ability of the Partnership and each of its subsidiaries to record, process, summarize and report financial data, or any material weaknesses in internal control over financial reporting and (B) any fraud, whether or not material, that involves management or other employees who have a significant role in the internal control over financial reporting of the Partnership and each of its subsidiaries, and (ii) there have been no changes in internal control over financial reporting, including any corrective actions with regard to significant deficiencies and material weaknesses, that has materially affected, or is reasonably likely to affect, the Partnership’s internal control over financial reporting;

(ff) the Partnership and its subsidiaries maintain “disclosure controls and procedures” (as such term is defined in Rule 13a-15(e) under the Exchange Act);

(gg) the Partnership is in compliance in all material respects with the applicable provisions of the Sarbanes-Oxley Act of 2002 and the rules and regulations promulgated in connection therewith, including Section 402 thereof related to loans to officers and

directors and Sections 302 and 906 related to certifications;

(hh) neither the Partnership nor any of its Subsidiaries nor, to the Partnership’s knowledge, any director, officer, agent, or employee of the Partnership or any of its Subsidiaries is aware of or has taken any action, directly or indirectly, that would result in a violation by such persons of the Foreign Corrupt Practices Act of 1977, as amended, and the rules and regulations thereunder, and the Partnership and its Subsidiaries have instituted and maintain policies and procedures designed to ensure, and which are reasonably expected to continue to ensure, continued compliance therewith;

(ii) the operations of the Partnership and its Subsidiaries are and have been conducted in compliance in all material respects with applicable financial recordkeeping and reporting requirements of the Currency and Foreign Transactions Reporting Act of 1970, as amended, and the money laundering statutes of all applicable jurisdictions (collectively, the “ Money Laundering Laws ”) and no action, suit or proceeding by or before any court or governmental agency, authority or body or any arbitrator involving the Partnership or any of its Subsidiaries with respect to the Money Laundering Laws is pending or, to the Partnership’s knowledge, threatened;

(jj) neither the Partnership nor any of its Subsidiaries nor, to the Partnership’s knowledge, any director, officer, agent, or employee of the Partnership or any of its Subsidiaries is currently subject to any U.S. sanctions administered by the Office of Foreign Assets Control of the U.S. Treasury Department (“ OFAC ”); and the Partnership will not directly or indirectly use the proceeds of the offering, or lend, contribute or otherwise make available such proceeds to any Subsidiary, joint venture partner or other person or entity, for the purpose of financing the activities of any person currently subject to any U.S. sanctions administered by OFAC;

(kk) no Subsidiary is currently prohibited, directly or indirectly, from paying any dividends or distributions to the Partnership, from repaying to the Partnership any loans or advances to such Subsidiary from the Partnership or from transferring any of such Subsidiary’s property or assets to the Partnership or any other Subsidiary of the Partnership, except (i) as set forth in Section 7.5 of the Partnership’s $1 billion amended and restated revolving credit agreement dated as of March 30, 2007, as modified by that certain Supplement and Joinder Agreement dated July 31, 2007 (collectively, the “ Credit Agreement ”), (ii) such limitations on transfer of equity interests in and assets of entities that are not wholly-owned by the Partnership or any of its subsidiaries and (iii) such prohibitions mandated by the laws of each such Subsidiary’s state of formation and the terms of any such Subsidiaries’ governing instruments;

(ll) the Partnership has not received any notice from the NYSE regarding the delisting of the Units from the NYSE;

(mm) the Partnership has not taken, directly or indirectly, any action designed to or that would constitute or that might reasonably be expected to cause or result in, under the Exchange Act or otherwise, stabilization or manipulation of the price of any security of the Partnership to facilitate the sale or resale of the Offered Units;

(nn) there is no contract or other document of a character required to be described in the Registration Statement or Basic Prospectus, or to be filed as an exhibit to the Registration Statement, which is not described or filed as required (and the Pre-Pricing Prospectus contains in all material respects the same description of the foregoing matters as will be contained in the Basic Prospectus, including the information incorporated by reference therein up to and through the date of this Agreement); and the statements in the Pre-Pricing Prospectus and the Prospectus under the heading “Material United States federal income tax considerations”, insofar as such statements summarize legal matters, agreements, documents or proceedings discussed therein, fairly summarize such legal matters, agreements, documents or proceedings in all material respects;

(oo) except as disclosed in the Disclosure Package and the Prospectus, the Partnership (i) does not have any material lending or other relationship with any bank or lending affiliate of an Underwriter and (ii) does not intend to use any of the proceeds from the sale of the Offered Units hereunder to repay any outstanding debt owed to any affiliate of an Underwriter;

(pp) the Subsidiaries listed on Annex A attached hereto are the only “significant subsidiaries” of the Partnership as of December 31, 2008 (as defined by Rule 1-02 of Regulation S-X); and

(qq) subsequent to the respective dates as of which information is given in the Registration Statement (as such information may have been superseded by a subsequent filing with the Commission), the Pre-Pricing Prospectuses, the Prospectus and the Permitted Free Writing Prospectuses, if any, in each case excluding any amendments or supplements to the foregoing made after the execution of this Agreement, there has not been (i) any material adverse change, or any development involving a prospective material adverse change, in the business, properties, management, financial condition or results of operations of the Partnership and the Subsidiaries taken as a whole, except for the sale of the Offered Units, (ii) any transaction which is material to the Partnership and the Subsidiaries, taken as a whole, (iii) any obligation or liability, direct or contingent (including any off-balance sheet obligations), incurred by the Partnership or any Subsidiary, which is material to the Partnership and the Subsidiaries, taken as a whole, except for the sale of the Offered Units and any drawdowns under the Credit Agreement, (iv) any change in the capitalization or outstanding indebtedness of the Partnership or any Subsidiaries, except for the sale of the Offered Units and any drawdowns under the Credit Agreement, or (v) any dividend or distribution of any kind declared, paid or made on the capital units of the Partnership or any Subsidiary.

In addition, any certificate signed by or on behalf of the Partnership and delivered to the Underwriters or counsel for the Underwriters in connection with the offering of the Offered Units shall be deemed to be a representation and warranty by the Partnership, as to matters covered thereby, to each Underwriter.

4. Certain Covenants of the Partnership . The Partnership hereby agrees:

(a) to furnish such information as may be required and otherwise to cooperate

in qualifying the Offered Units for offering and sale under the securities or blue sky laws of such states or other jurisdictions as you may designate and to maintain such qualifications in effect so long as you may reasonably request for the distribution of the Offered Units; provided , however , that the Partnership shall not be required to qualify as a foreign corporation or to consent to the service of process under the laws of any such jurisdiction (except service of process with respect to the offering and sale of the Offered Units); and to promptly advise you of the receipt by the Partnership of any notification with respect to the suspension of the qualification of the Offered Units for offer or sale in any jurisdiction or the initiation or threatening of any proceeding for such purpose;

(b) to make available to the Underwriters in New York City, as soon as practicable after this Agreement becomes effective, and thereafter from time to time to furnish to the Underwriters, as many copies of the Prospectus (or of the Prospectus as amended or supplemented if the Partnership shall have made any amendments or supplements thereto after the effective date of the Registration Statement) as the Underwriters may reasonably request for the purposes contemplated by the Act; in case any Underwriter is required to deliver (whether physically or through compliance with Rule 172 under the Act or any similar rule), in connection with the sale of the Offered Units, a prospectus after the nine-month period referred to in Section 10(a)(3) of the Act, or after the time a post-effective amendment to the Registration Statement is required pursuant to Item 512(a) of Regulation S-K under the Act, the Partnership will prepare, at its expense, promptly upon request such amendment or amendments to the Registration Statement and the Prospectus as may be necessary to permit compliance with the requirements of Section 10(a)(3) of the Act or Item 512(a) of Regulation S-K under the Act, as the case may be;

(c) if, at the time this Agreement is executed and delivered, it is necessary or appropriate for a post-effective amendment to the Registration Statement to be filed with the Commission and become effective before the Offered Units may be sold, the Partnership will use its best efforts to cause such post-effective amendment or such Registration Statement to be filed and become effective, and will pay any applicable fees in accordance with the Act, as soon as possible; and the Partnership will advise you promptly and, if requested by you, will confirm such advice in writing, (i) when such post-effective amendment or such Registration Statement has become effective, and (ii) if Rule 430A under the Act is used, when the Prospectus is filed with the Commission pursuant to Rule 424(b) under the Act (which the Partnership agrees to file in a timely manner in accordance with such Rules);

(d) if, at any time during the period when a prospectus is required by the Act to be delivered (whether physically or through compliance with Rule 172 under the Act or any similar rule) in connection with any sale of Offered Units, the Registration Statement shall cease to comply with the requirements of the Act with respect to eligibility for the use of the form on which the Registration Statement was filed with the Commission or the Registration Statement shall cease to be an “automatic shelf registration statement” (as defined in Rule 405 under the Act) or the Partnership shall have received, from the Commission, a notice, pursuant to Rule 401(g)(2), of objection to the use of the form on which the Registration Statement was filed with the Commission,

to (i) promptly notify you, (ii) promptly file with the Commission a new registration statement under the Act, relating to the Offered Units, or a post-effective amendment to the Registration Statement, which new registration statement or post-effective amendment shall comply with the requirements of the Act and shall be in a form reasonably satisfactory to you, (iii) use its best efforts to cause such new registration statement or post-effective amendment to become effective under the Act as soon as practicable, (iv) promptly notify you of such effectiveness and (v) take all other action necessary or appropriate to permit the public offering and sale of the Offered Units to continue as contemplated in the Prospectus; all references herein to the Registration Statement shall be deemed to include each such new registration statement or post-effective amendment, if any;

(e) if the third anniversary of the initial effective date of the Registration Statement (within the meaning of Rule 415(a)(5) under the Act) shall occur at any time during the period when a prospectus is required by the Act to be delivered (whether physically or through compliance with Rule 172 under the Act or any similar rule) in connection with any sale of Offered Units, to file with the Commission, prior to such third anniversary, a new registration statement under the Act relating to the Offered Units, which new registration statement shall comply with the requirements of the Act (including, without limitation, Rule 415(a)(6) under the Act) and shall be in a form reasonably satisfactory to you; such new registration statement shall constitute an “automatic shelf registration statement” (as defined in Rule 405 under the Act); provided , however , that if the Partnership is not then eligible to file an “automatic shelf registration statement” (as defined in Rule 405 under the Act), then such new registration statement need not constitute an “automatic shelf registration statement” (as defined in Rule 405 under the Act), but the Partnership shall use its best efforts to cause such new registration statement to become effective under the Act as soon as practicable, but in any event within 180 days after such third anniversary and promptly notify you of such effectiveness; the Partnership shall take all other action necessary or appropriate to permit the public offering and sale of the Offered Units to continue as contemplated in the Prospectus; all references herein to the Registration Statement shall be deemed to include each such new registration statement, if any;

(f) to advise you promptly, confirming such advice in writing, of any request by the Commission for amendments or supplements to the Registration Statement, any Pre-Pricing Prospectus, the Prospectus or any Permitted Free Writing Prospectus or for additional information with respect thereto, or of notice of institution of proceedings for, or the entry of a stop order, suspending the effectiveness of the Registration Statement and, if the Commission should enter a stop order suspending the effectiveness of the Registration Statement, to use its best efforts to obtain the lifting or removal of such order as soon as possible; to advise you promptly of any proposal to amend or supplement the Registration Statement, any Pre-Pricing Prospectus or the Prospectus, and to provide you and Underwriters’ counsel copies of any such documents for review and comment a reasonable amount of time prior to any proposed filing and to file no such amendment or supplement to which you shall reasonably object in writing, unless, (i) in the judgment of counsel to the Partnership, such filing is required by applicable law or (ii) is advisable in furtherance of a Commission request;

(g) subject to Section 4(f) hereof, to file promptly all reports and documents and any preliminary or definitive proxy or information statement required to be filed by the Partnership with the Commission in order to comply with the Exchange Act for so long as a prospectus is required by the Act to be delivered (whether physically or through compliance with Rule 172 under the Act or any similar rule) in connection with any sale of Offered Units; and, for 30 days from the date hereof so long as the Underwriters have informed the Partnership that they have not completed their distribution of the Offered Units, to provide you, for your review and comment, with a copy of such reports and statements and other documents to be filed by the Partnership pursuant to Section 13, 14 or 15(d) of the Exchange Act during such period a reasonable amount of time prior to any proposed filing, and to file no such report, statement or document to which you shall have reasonably objected in writing; and to promptly notify you of such filing;

(h) to pay the fees applicable to the Registration Statement in connection with the offering of the Offered Units within the time required by Rule 456(b)(1)(i) under the Act (without reliance on the proviso to Rule 456(b)(1)(i) under the Act) and in compliance with Rule 456(b) and Rule 457(r) under the Act;

(i) to advise the Underwriters promptly of the happening of any event within the period during which a prospectus is required by the Act to be delivered (whether physically or through compliance with Rule 172 under the Act or any similar rule