UNIT PURCHASE AGREEMENT

Exhibit  10.1

UNIT PURCHASE AGREEMENT

by and among

MARKWEST ENERGY PARTNERS, L.P.,

MARKWEST ENERGY GP, L.L.C.

AND

THE PURCHASERS PARTY HERETO

UNIT PURCHASE AGREEMENT

UNIT PURCHASE AGREEMENT, dated as of December 18, 2007 (this “ Agreement ”), by and among MARKWEST ENERGY PARTNERS, L.P. (“ MarkWest ”), MARKWEST ENERGY GP, L.L.C. (“ MarkWest GP ”) (solely for purposes of Section 6.12) and each of the purchasers set forth on Schedule 2.02 hereof (each a “ Purchaser ” and collectively, the “ Purchasers ”).

In consideration of the mutual covenants and agreements set forth herein and for good and valuable consideration, the receipt of which is hereby acknowledged, the parties hereby agree as follows:

ARTICLE I
DEFINITIONS

Section 1.01           Definitions .  As used in this Agreement, and unless the context requires a different meaning, the following terms have the meanings indicated:

“ Action ” against a Person means any lawsuit, action, proceeding, investigation or complaint before any Governmental Authority, mediator or arbitrator.

“ Affiliate ” means, with respect to a specified Person, any other Person, directly or indirectly controlling, controlled by or under direct or indirect common control with such specified Person.  For purposes of this definition, “control” (including, with correlative meanings, “controlling,” “controlled by,” and “under common control with”) means the power to direct or cause the direction of the management and policies of such Person, directly or indirectly, whether through the ownership of voting securities, by contract or otherwise.

“ Agreement ” has the meaning set forth in the introductory paragraph.

“ Basic Documents ” means, collectively, this Agreement, the Registration Rights Agreement, and any and all other agreements or instruments executed and delivered to the Purchasers by MarkWest or any Subsidiary of MarkWest hereunder or thereunder.

“ Business Day ” means any day other than a Saturday, Sunday, or a legal holiday for commercial banks in New York, New York.

“ Closing ” shall have the meaning specified in Section 2.02.

“ Closing Date ” shall have the meaning specified in Section 2.02.

“ Commission ” means the United States Securities and Exchange Commission.

“ Common Units ” means the common units of MarkWest.

“ Confidential Information ” means all oral or written information, documents, records and data that MarkWest or its Representatives furnishes or otherwise discloses to a Purchaser or any of its Representatives, together with all copies, extracts, analyses, compilations, studies,

memoranda, notes or other documents, records or data (in whatever form maintained, whether documentary, computer or other electronic storage or otherwise) prepared by any Person that contain or otherwise reflect or are generated from such information, documents, records, or data.  The term “Confidential Information” does not include any information that (a) at the time of disclosure or thereafter is generally available to the public (other than as a result of a disclosure by such Purchaser or its Representatives), (b) is developed by such Purchaser or any of its Representatives, independent of, and without reliance in whole or in part on, any Confidential Information or any knowledge of Confidential Information, (c) becomes available to such Purchaser or its Representatives on a non-confidential basis from a source other than MarkWest or its Representatives who, insofar as is known to the recipient after reasonable inquiry, is not prohibited from transmitting the information to the recipient by a contractual, legal, fiduciary or other obligation to MarkWest or (d) was available to such Purchaser or its Representatives on a non-confidential basis prior to its disclosure to such Purchaser or its Representatives by MarkWest or its Representatives.

“ Exchange Act ” means the Securities Exchange Act of 1934, as amended from time to time, and the rules and regulations of the Commission promulgated thereunder.

“ Form 8-A ” shall have the meaning set forth in Section 3.02(a).

“ GAAP ” means generally accepted accounting principles in the United States of America in effect from time to time.

“ Governmental Authority ” means, with respect to a particular Person, the country, state, county, city and political subdivisions in which such Person or such Person’s Property is located or which exercises valid jurisdiction over any such Person or such Person’s Property, and any court, agency, department, commission, board, bureau or instrumentality of any of them and any monetary authority which exercises valid jurisdiction over any such Person or such Person’s Property.  Unless otherwise specified, all references to Governmental Authority herein with respect to MarkWest means a Governmental Authority having jurisdiction over MarkWest, its Subsidiaries or any of their respective Properties.

“ Hydrocarbon Acquisition ” means the transactions contemplated by the Hydrocarbon Purchase Agreement.

“ Hydrocarbon Purchase Agreement ” means the Agreement and Plan of Redemption and Merger, dated as of September 5, 2007 by and among MarkWest Hydrocarbon, Inc., MarkWest and MWEP, L.L.C.

“ Indemnified Party ” shall have the meaning specified in Section 6.02(c).

“ Indemnifying Party ” shall have the meaning specified in Section 6.02(c).

“ Knowledge ” means the actual knowledge of the individuals listed on Schedule 1.01 hereto after reasonable inquiry.

“ Law ” means any federal, state, local or foreign order, writ, injunction, judgment, settlement, award, decree, statute, law, rule or regulation.

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“ Lien ” means any lien, encumbrance, security interest, charge or other interest in Property securing an obligation owed to, or a claim by, a Person other than the owner of the Property, whether such interest is based on the common law, statute or contract, and whether such obligation or claim is fixed or contingent, and including but not limited to the lien or security interest arising from a mortgage, encumbrance, pledge, security agreement, conditional sale or trust receipt or a lease, consignment or bailment for security purposes. For the purpose of this Agreement, a Person shall be deemed to be the owner of any Property which it has acquired or holds subject to a conditional sale agreement, or leases under a financing lease or other arrangement pursuant to which title to the Property has been retained by or vested in some other Person in a transaction intended to create a financing.

“ MarkWest ” has the meaning set forth in the introductory paragraph.

“ MarkWest Financial Statements ” means the financial statement or statements described or referred to in Section 3.03.

“ MarkWest GP ” has the meaning set forth in the introductory paragraph.

“ MarkWest Material Adverse Effect ” means any material and adverse effect on (a) the assets, liabilities, financial condition, business, operations or affairs of MarkWest and its Subsidiaries taken as a whole measured against those assets, liabilities, financial condition, business, operations or affairs reflected in the MarkWest SEC Documents filed with the Commission prior to the date hereof or from the facts represented or warranted in any Basic Document, or (b) the ability of MarkWest to fulfill any of its obligations under or consummate any of the transactions contemplated by the Basic Documents.

“ MarkWest Related Parties ” shall have the meaning specified in Section 6.02(b).

“ MarkWest SEC Documents ” shall have the meaning specified in Section 3.03.

“ Partnership Agreement ” means the Second Amended and Restated Agreement of Limited Partnership of MarkWest, dated as of February 28, 2007.

“ Partnership Securities ” means any class or series of equity interest in MarkWest (but excluding any options, rights, warrants and appreciation rights relating to an equity interest in MarkWest), including without limitation Common Units and Incentive Distribution Rights (as defined in the Partnership Agreement).

“ Permits ” means, with respect to MarkWest or any of its Subsidiaries, any licenses, permits, variances, consents, authorizations, waivers, grants, franchises, concessions, exemptions, orders, registrations and approvals of Governmental Authorities or other Persons necessary for the ownership, leasing, operation, occupancy and use of its Properties and the conduct of its businesses as currently conducted.

“ Person ” means any individual, corporation, company, voluntary association, partnership, joint venture, trust, limited liability company, unincorporated organization or government or any agency, instrumentality or political subdivision thereof, or any other form of entity.

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“ Property ” means any interest in any kind of property or asset, whether real, personal or mixed, or tangible or intangible.

“ Purchase Price ” means, with respect to a particular Purchaser, the monetary purchase amount set forth opposite such Purchaser’s name under the column entitled “Total Purchase Price” on Schedule 2.02 hereto.

“ Purchased Units ” means, with respect to a particular Purchaser, the number of Common Units set forth opposite such Purchaser’s name under the column entitled “Units Purchased” set forth on Schedule 2.02 hereto.

“ Purchaser ” and “ Purchasers ” each has the meaning set forth in the introductory paragraph.

“ Purchaser Material Adverse Effect ” means, with respect to a particular Purchaser, any material and adverse effect on (a) the assets, liabilities, financial condition, business, operations or affairs of such Purchaser, (b) the ability of such Purchaser to carry out its business as of the date hereof or to meet its obligations under the Basic Documents on a timely basis or (c) the ability of such Purchaser to consummate the transactions under any Basic Document.

“ Purchaser Related Parties ” shall have the meaning specified in Section 6.02(a).

“ Registration Rights Agreement ” means the Registration Rights Agreement, to be entered into at the Closing, among MarkWest and the Purchasers in the form attached hereto as Exhibit A.

“ Representatives ” of any Person means the officers, directors, employees, agents, counsel, investment bankers and other representatives of such Person.

“ Securities Act ” means the Securities Act of 1933, as amended from time to time, and the rules and regulations of the Commission promulgated thereunder.

“ Short Sales ” means, without limitation, all “short sales” as defined in Rule 200 promulgated under Regulation SHO under the Exchange Act, whether or not against the box, and forward sale contracts, options, puts, calls, short sales, “put equivalent positions” (as defined in Rule 16a-1(h) under the Exchange Act) and similar arrangements, and sales and other transactions through non-U.S. broker dealers or foreign regulated brokers.

“ Subsidiary ” means, as to any Person, any corporation or other entity of which: (i) such Person or a Subsidiary of such Person is a general partner or manager; or (ii) at least a majority of the outstanding equity interest having by the terms thereof ordinary voting power to elect a majority of the board of directors or similar governing body of such corporation or other entity (irrespective of whether or not at the time any equity interest of any other class or classes of such corporation or other entity shall have or might have voting power by reason of the happening of any contingency) is at the time directly or indirectly owned or controlled by such Person or one or more of its Subsidiaries.

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“ Tax ” means any tax, charge, fee, levy, penalty or other assessment imposed by any U.S. federal, state, local or foreign taxing authority, including any excise, property, income, sales, transfer, franchise, payroll, withholding, social security or other tax, including any interest, penalties or additions attributable thereto.

“ Tax Return ” means any return, report, information return, declaration, claim for refund or other document (including any related or supporting information) supplied or required to be supplied to any authority with respect to Taxes and including any supplement or amendment thereof.

“ Unit Price ” shall have the meaning set forth in Section 2.04.

Section 1.02           Accounting Procedures and Interpretation .  Unless otherwise specified herein, all accounting terms used herein shall be interpreted, all determinations with respect to accounting matters hereunder shall be made, and all MarkWest Financial Statements and certificates and reports as to financial matters required to be furnished to the Purchasers hereunder shall be prepared, in accordance with GAAP applied on a consistent basis during the periods involved (except, in the case of unaudited statements, as permitted by Form 10-Q promulgated by the Commission) and in compliance as to form in all material respects with applicable accounting requirements and with the published rules and regulations of the Commission with respect thereto.

ARTICLE II
AGREEMENT TO SELL AND PURCHASE

Section 2.01           Sale and Purchase .  Subject to the terms and conditions hereof, at the Closing (as defined in Section 2.02) MarkWest hereby agrees to issue and sell to each Purchaser, and each Purchaser hereby agrees to purchase from MarkWest, such Purchaser’s Purchased Units, and each Purchaser agrees to pay MarkWest such Purchaser’s Purchase Price. The obligation of each Purchaser hereunder is several and not joint and is independent of the obligation of each other Purchaser, and the failure of, or MarkWest’s waiver of, performance by any Purchaser does not excuse performance by any other Purchaser or MarkWest.

Section 2.02           Closing .  Subject to the terms and conditions hereof, the consummation of the purchase and sale of the Purchased Units hereunder (the “ Closing ”) shall take place on December 18, 2007 (such date, the “ Closing Date ”) at the offices of Vinson & Elkins, L.L.P., 1001 Fannin Street, Suite 2500, Houston, Texas 77002.  Each of the items to be delivered, as set forth in Section 2.03, shall be delivered by the responsible party on the Closing Date.

Section 2.03           Deliveries .

(a)           At the Closing, subject to the terms and conditions hereof, MarkWest will deliver, or cause to be delivered, to each Purchaser:

(i)                    The Purchased Units to be purchased by such Purchaser by delivery of certificates evidencing such Purchased Units at the Closing meeting the requirements of the Partnership Agreement, all free and clear of any Liens, encumbrances or interests of any other Person;

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(ii)                   A certificate of the Secretary of State of the State of Delaware, dated as of a recent date, that each of MarkWest, MarkWest GP and MarkWest Energy Operating Company, L.L.C. is in good standing;

(iii)                  An opinion addressed to the Purchasers from Vinson & Elkins L.L.P., dated as of the Closing Date, in the form and substance attached hereto as Exhibit C;

(iv)                  The Registration Rights Agreement, which shall have been duly executed by MarkWest.

(b)           At the Closing, subject to the terms and conditions hereof, each Purchaser will deliver, or cause to be delivered to MarkWest.

(i)                    The Registration Rights Agreement, which shall have been duly executed by each such Purchaser; and

(ii)                   Such Purchaser’s Purchase Price by wire transfer of immediately available funds to an account designated by MarkWest in writing prior to the Closing.

Section 2.04           Consideration .  The amount per Common Units each Purchaser will pay to MarkWest to purchase the Common Units comprising the Purchased Units shall be $31.50 per Common Unit (the “ Unit Price ”).

Section 2.05           Independent Nature of Purchasers’ Obligations and Rights .   The obligations of each Purchaser under any Basic Document are several and not joint with the obligations of any other Purchaser, and no Purchaser shall be responsible in any way for the performance of the obligations of any other Purchaser under any Basic Document.  Nothing contained herein or in any Basic Document, and no action taken by any Purchaser pursuant thereto, shall be deemed to constitute the Purchasers as a partnership, an association, a joint venture or any other kind of entity, or create a presumption that the Purchasers are in any way acting in concert or as a group with respect to such obligations or the transactions contemplated by the Basic Document.  Each Purchaser shall be entitled to independently protect and enforce its rights, including without limitation, the rights arising out of this Agreement or out of the other Basic Documents, and it shall not be necessary for any other Purchaser to be joined as an additional party in any proceeding for such purpose.  Each Purchaser has been represented by its own separate legal counsel in their review and negotiation of the Basic Documents.   MarkWest has elected to provide all Purchasers with the same material terms and Basic Documents for the convenience of MarkWest and not because it was required or requested to do so by the Purchasers.

Section 2.06           Lock-up .

(a)           MarkWest agrees that from and after Closing, other than (i) in connection with the Hydrocarbon Acquisition or (ii) pursuant to MarkWest’s Long-Term Incentive Plan and MarkWest’s other equity compensation plans, as described in MarkWest’s Annual Report on Form 10-K for the period ended December 31, 2006, it will not sell,

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nor will it permit any of its Affiliates to offer, sell, contract to sell, pledge or otherwise dispose of, or enter into any transaction which is designed to, or might reasonably be expected to, result in the disposition of any Common Units prior to the date which is 90 days from the Closing Date.

(b)           Each Purchaser agrees that from and after Closing it will not offer, sell, contract to sell, pledge or otherwise dispose of, or enter into any transaction which is designed to, or might reasonably be expected to, result in the disposition of any of the Purchased Units prior to the date which is 90 days from the Closing Date; provided , however , that any Purchaser may (i) pledge its Purchased Units in connection with a bona fide margin account or other loan or financing arrangement secured by such Purchaser’s Purchased Units or (ii) enter into a cash settled derivative, total return swap or similar transaction, in each case, with respect to the Purchased Units purchased by it.

ARTICLE III
REPRESENTATIONS AND WARRANTIES RELATED TO MARKWEST

MarkWest represents and warrants to each Purchaser as follows:

Section 3.01           Corporate Existence .  MarkWest: (a) is a limited partnership duly organized, legally existing and in good standing under the laws of the State of Delaware; and (b) has all requisite power and authority, and has all governmental licenses, authorizations, consents and approvals necessary, to own, lease, use and operate its Properties and carry on its business as its business is now being conducted, except where the failure to obtain such licenses, authorizations, consents and approvals would not be reasonably likely to have a MarkWest Material Adverse Effect.  Each of MarkWest’s Subsidiaries that is a corporation is a corporation duly incorporated, validly existing and in good standing under the laws of the State or other jurisdiction of its incorporation and has all requisite power and authority, and has all governmental licenses, authorizations, consents and approvals necessary, to own, lease, use or operate its respective Properties and carry on its business as now being conducted, except where the failure to obtain such licenses, authorizations, consents and approvals would not be reasonably likely to have a MarkWest Material Adverse Effect.  Each Subsidiary of MarkWest that is not a corporation has been duly formed, is validly existing and in good standing under the laws of the State or other jurisdiction of its formation and has all requisite power and authority, and has all governmental licenses, authorizations, consents and approvals necessary, to own, lease, use or operate its respective Properties and carry on its business as now being conducted, except where the failure to obtain such licenses, authorizations, consents and approvals would not be reasonably likely to have a MarkWest Material Adverse Effect.  None of MarkWest or any of its Subsidiaries are in default in the performance, observance or fulfillment of any provision of, in the case of MarkWest, the Partnership Agreement or its Certificate of Limited Partnership or, in the case of any Subsidiary of MarkWest, its respective certificate of incorporation, bylaws or other similar organizational documents.  Each of MarkWest and its Subsidiaries is duly qualified or licensed and in good standing as a foreign corporation or other entity, and is authorized to do business, in each jurisdiction in which the ownership or leasing of its respective Properties or the character of its respective operations makes such qualification necessary, except where the failure to obtain such qualification, license, authorization or good standing would not be reasonably likely to have a MarkWest Material Adverse Effect.

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Section 3.02           Capitalization and Valid Issuance of Purchased Units .

(a)           As of the date hereof, the issued and outstanding limited partner interests of MarkWest consist of 36,500,445 Common Units.  The only issued and outstanding general partner interests of MarkWest are the interests of the General Partner described and as defined in the Partnership Agreement.  All outstanding Common Units and the limited partner interests represented thereby have been duly authorized and validly issued in accordance with the Partnership Agreement and are fully paid (to the extent required under the Partnership Agreement) and nonassessable (except as such nonassessability may be affected by matters described under the caption “The Partnership Agreement-Limited Liability” in MarkWest’s registration statement on Form S-1 (No. 333-81780) which is incorporated by reference into the Partnership’s Registration Statement on Form 8-A (File No. 001-31239) (the “ Form 8-A ”)).

(b)           Other than MarkWest’s Long-Term Incentive Plan and MarkWest’s other equity compensation plans, as described in MarkWest’s Annual Report on Form 10-K for the period ended December 31, 2006, MarkWest has no equity compensation plans that contemplate the issuance of Common Units (or securities convertible into or exchangeable for Common Units).  No indebtedness having the right to vote (or convertible into or exchangeable for securities having the right to vote) on any matters on which MarkWest unitholders may vote is issued or outstanding.  Except as set forth in the first sentence of this Section 3.02(b), as are contained in the Partnership Agreement or as contemplated by the Hydrocarbon Purchase Agreement and disclosed in the joint proxy statement/prospectus that is part of the Registration Statement on Form S-4 filed with the Commission on November 11, 2007 (File No. 333-147370), there are no outstanding or authorized (i) options, warrants, preemptive rights, subscriptions, calls, or other rights, convertible securities, agreements, claims or commitments of any character obligating MarkWest or any of its Subsidiaries to issue, transfer or sell any partnership interests or other equity interest in, MarkWest or any of its Subsidiaries or securities convertible into or exchangeable for such partnership interests or equity interests, (ii) obligations of MarkWest or any of its Subsidiaries to repurchase, redeem or otherwise acquire any partnership interests or equity interests of MarkWest or any of its Subsidiaries or any such securities or agreements listed in clause (i) of this sentence or (iii) voting trusts or similar agreements to which MarkWest or any of its Subsidiaries is a party with respect to the voting of the equity interests of MarkWest or any of its Subsidiaries.  At the Closing, except as described in this Section 3.02(b) and as contemplated by the Hydrocarbon Purchase Agreement, there will not be any outstanding subscriptions, options, warrants, calls, preemptive rights, subscriptions, or other rights, convertible or exchangeable securities, agreements, claims or commitments of any character by which MarkWest or any of its Subsidiaries will be bound calling for the purchase or issuance of any partnership interests of MarkWest or any equity interest of any of its Subsidiaries or securities convertible into or exchangeable for such partnership or equity interests or any other such securities or agreements.  Neither the offering or sale of the Purchased Units nor registration of the Purchased Units pursuant to the Registration Rights Agreement gives rise to any rights for or relating to the registration of any Common Units or other securities of MarkWest.

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(c)           (i) All of the issued and outstanding equity interests of each of MarkWest’s Subsidiaries are owned, directly or indirectly, by MarkWest free and clear of any Liens (except for such restrictions as may exist under applicable Law and except for such Liens as may be imposed under MarkWest’s or MarkWest’s Subsidiaries’ credit facilities), and all such ownership interests have been duly authorized, validly issued and are fully paid (to the extent required in the organizational documents of MarkWest’s Subsidiaries, as applicable) and non-assessable (except as nonassessability may be affected by Section 6.07 of the Texas Revised Uniform Limited Partnership Act,  Section 18-607 of the Delaware Limited Liability Company Act, Section 17-607 of the Delaware Revised Uniform Limited Partnership Act, Section 450.4307 of the Michigan Limited Liability Company Act, Section 2030 of the Oklahoma Limited Liability Company Act or the organizational documents of MarkWest’s Subsidiaries, as applicable) and free of preemptive rights, with no personal liability attaching to the ownership thereof, and (ii) as of the date hereof, neither MarkWest nor any of its Subsidiaries owns any shares of capital stock or other securities of, or interest in, any other Person, or is obligated to make any capital contribution to or other investment in any other Person.  The material Subsidiaries of MarkWest are set forth on Schedule 3.02 hereto.

(d)           The Common Units being purchased by the Purchasers hereunder and the limited partner interests represented thereby, are duly authorized by the Partnership Agreement and, when issued and delivered to the Purchasers against payment therefor in accordance with the terms of this Agreement, will be validly issued, fully paid (to the extent required by the Partnership Agreement) and nonassessable (except as such nonassessability may be affected by matters described under the caption “The Partnership Agreement—Limited Liability” in MarkWest’s registration statement on Form S-1 (No. 333-81780) which is incorporated by reference into the Form 8-A) and will be free of any and all Liens and restrictions on transfer, other than restrictions on transfer under the Partnership Agreement and under applicable state and federal securities laws and other than such Liens as are created by the Purchaser.

(e)           The Common Units are listed on the New York Stock Exchange.

(f)            Correct and complete copies of the Partnership Agreement and MarkWest’s certificate of limited partnership are attached hereto as Exhibit B.

Section 3.03           MarkWest SEC Documents .  MarkWest has filed with the Commission all forms, registration statements, reports, schedules and statements required to be filed by it under the Exchange Act or the Securities Act (all such documents, collectively “ MarkWest SEC Documents ”) and, since October 11, 2005, all such reports and statements have been timely filed.  The MarkWest SEC Documents, including, without limitation, any audited or unaudited financial statements and any notes thereto or schedules included therein (the “ MarkWest Financial Statements ”), at the time filed (in the case of registration statements, solely on the dates of effectiveness) (except to the extent corrected by a subsequently filed MarkWest SEC Document filed prior to the date hereof) (a) did not contain any untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary in order to make the statements therein, in light of the circumstances under which they were made, not misleading, (b) 

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complied in all material respects with the applicable requirements of the Exchange Act and the Securities Act, as the case may be, (c) complied as to form in all material respects with applicable accounting requirements and with the published rules and regulations of the Commission with respect thereto, (d) were prepared in accordance with GAAP applied on a consistent basis during the periods involved (except as may be indicated in the notes thereto or, in the case of unaudited statements, as permitted by Form 10-Q of the Commission), and (e) fairly present (subject in the case of unaudited statements to normal, recurring and year-end audit adjustments) in all material respects the consolidated financial position and status of the business of MarkWest as of the dates thereof and the consolidated results of its operations and cash flows for the periods then ended.  PricewaterhouseCoopers LLP, MarkWest’s former public accounting firm, is an independent public accounting firm with respect to MarkWest and did not resign and was not dismiss