MEMBERSHIP INTEREST SALE AGREEMENT

EXHIBIT 2.2

EXECUTION VERSION

MEMBERSHIP INTEREST SALE AGREEMENT

     This Membership Interest Sale Agreement (the “ Agreement ”) is executed as of May 6, 2008 between Pioneer Natural Resources USA, Inc., a Delaware corporation (“ Pioneer USA ”), and Pioneer Southwest Energy Partners L.P., a Delaware limited partnership (“ MLP ”, and together with Pioneer USA, being sometimes collectively referred to as the “ Parties ” and individually as a “ Party ”).

RECITALS:

     A. Pioneer USA is the sole member and holds 100% of the membership interest in Pioneer Southwest Energy Partners USA LLC, a Texas limited liability company (the “ Operating Company ”), pursuant to that certain Limited Liability Company Agreement dated October 7, 2007.

     B. The Merger has been consummated.

     C. Pioneer USA and MLP desire for MLP, pursuant to the transactions to be effected pursuant to this Agreement and the Contribution Agreement, to acquire 100% of the membership interest in the Operating Company (the “ Acquisition ”).

     D. The Acquisition will be consummated immediately after the issuance of the IPO Units by MLP.

     E. MLP wishes to be admitted to the Operating Company as the sole member of the Operating Company.

DEFINITIONS

     The following capitalized terms have the meanings given below:

          “ AAA ” means the American Arbitration Association.

          “ Affiliate ” means, with respect to any Person, any other Person that directly or indirectly through one or more intermediaries controls, is controlled by or is under common control with the Person in question. As used herein, the term “control” means the possession, direct or indirect, of the power to direct or cause the direction of the management and policies of a Person, whether through ownership of Voting Securities, by contract or otherwise.

          “ Agreement ” has the meaning assigned to such term in the first paragraph of this Agreement.

          “ Assignment and Assumption Agreement ” means the form of Assignment and Assumption Agreement attached hereto as Exhibit A.

          “ Closing ” means the closing of the purchase and sale of the Interest.

          “ Contribution Agreement ” means the Contribution Agreement by and among Pioneer USA, Pioneer Natural Resources GP LLC and MLP dated May 6, 2008, which agreement is being entered into simultaneously with the execution of this Agreement.

          “ Dispute ” has the meaning assigned to such term in Section 20 of this Agreement.

          “ Interest ” means a 26.5396546169060% membership interest in the Operating Company.

          “ IPO Units ” means 8,250,000 common units representing limited partner interests in MLP to be issued by MLP to the underwriters in connection with MLP’s initial public offering of such common units.

          “ Merger ” has the meaning assigned to such term in the Merger Agreement.

          “ Merger Agreement ” means the Agreement and Plan of Merger, dated as of May 1, 2008, among Pioneer USA, the Operating Company, Pioneer Retained Properties Company LLC, and Pioneer Limited Natural Resources Properties LLC.

          “ MLP ” has the meaning assigned to such term in the first paragraph of this Agreement.

          “ Operating Company ” has the meaning assigned to such term in Recital A of this Agreement.

          “ Party ” or “ Parties ” have the meanings assigned to such terms in the first paragraph of this Agreement.

          “ Person ” means an individual or a corporation, limited liability company, partnership, joint venture, trust, unincorporated organization, association, government agency or political subdivision thereof or other entity.

          “ Pioneer USA ” has the meaning assigned to such term in the first paragraph of this Agreement.

          “ Representatives ” means, with respect to a particular Person, any director, officer, employee, agent, consultant, advisor, or other representative of such Person, including legal counsel, accountants, and financial advisors.

          “ Rules ” means the Commercial Arbitration Rules of the AAA.

          “ Voting Securities ” means securities of any class of a Person entitling the holders thereof to vote in the election of, or to appoint, members of the board of directors or other similar governing body of the Person and, with respect to MLP, means common units representing limited partnership interests in MLP.

AGREEMENTS

     For valuable consideration, whose receipt and sufficiency are hereby acknowledged, the Parties agree as follows:

     1.  Purchase and Sale; Assumption . Pursuant to the terms of this Agreement, Pioneer USA hereby agrees to sell to MLP, and MLP hereby agrees to purchase from Pioneer USA, all of Pioneer USA’s right, title, and interest in and to the Interest. Pursuant to the terms of this Agreement, MLP hereby agrees to assume the obligations attributable to the Interest that arise on and after the Closing. At the Closing, Pioneer USA and MLP agree to execute and deliver to each other the Assignment and Assumption Agreement.

     2.  Payment and Closing . In consideration of the sale of the Interest by Pioneer USA to MLP, MLP, at the Closing, shall pay to Pioneer USA $141,061,250 in cash by wire transfer to an account designated by Pioneer USA. The Closing will occur on the date of this Agreement. The Closing and the transactions described in Section 2.2 of the Contribution Agreement shall all occur simultaneously and as component parts of a single plan.

     3.  Admission . Pursuant to the Assignment and Assumption Agreement, MLP shall be admitted as the sole member and owner of 100% of the membership interest in the Operating Company, and MLP’s admission as the sole member of the Operating Company shall be reflected in the books and records of the Operating Company.

     4.  Tax Deferred Exchange. If Pioneer USA so requests, MLP agrees to cooperate with Pioneer USA in a tax-deferred exchange described in Section 1031 of the Internal Revenue Code of 1986, as amended, with respect to the transaction described in this Agreement. In that case, the Assignment and Assumption Agreement will be executed by Pioneer USA’s assignee unless such assignee directs Pioneer USA to execute the Assignment and Assumption Agreement. Notwithstanding the foregoing, MLP shall not be obligated to enter into any agreement or to consent to an assignment of Pioneer USA’s rights or obligations hereunder which may have the effect of (i) impairing the title to the Interest, (ii) increasing MLP’s obligations or liability hereunder or resulting in any additional cost, expense, or liability to MLP, or (iii) requiring MLP to execute a purchase agreement for the purchase of the exchange property or to take record title to the exchange property. Pioneer USA hereby agrees to indemnify, defend, and hold MLP harmless from and against any and all costs, expenses, claims, damages, losses, or liabilities (including reasonable attorney fees and costs) incurred by MLP in connection with any exchange transaction or transactions or the performance by MLP of its obligations pursuant to this Section 4.

     5.  Investment Intent . The Interest to be acquired by MLP is being acquired for its own account, for investment and with no intention of distributing or reselling such Interest or any part thereof or interest therein in any transaction which would be a violation of the securities laws of the United States of America or any state or foreign country or jurisdiction.

     6.  No Representations and Warranties . Pioneer USA makes no representations or warranties whatsoever, and disclaims all liability and responsibility for any representation, warranty, statement, or information made or communicated (orally or in writing) to MLP

(including any opinion, information, or advice which may have been provided to MLP by any Affiliate or Representative of Pioneer USA or by any investment bank or investment banking firm, any petroleum engineer or engineering firm, Pioneer USA’s counsel, or any other agent, consultant, or representative) regarding the Interest or the assets, liabilities or operations of the Operating Company.

     7.  Choice of Law . This Agreement shall be subject to and governed by the laws of the State of Texas, excluding any conflicts-of-law rule or principle that might refer the construction or interpretation of this Agreement to the laws of another state.

     8.  Notice . All notices, requests or consents provided for or permitted to be given pursuant to this Agreement must be in writing and must be given by depositing same in the United States mail, addressed to the Person to be notified, postpaid, and registered or certified with return receipt requested or by delivering such notice in person or by telecopier or telegram to such Party. Notice given by personal delivery or mail shall be effective upon actual receipt. Notice given by telegram or telecopier shall be effective upon actual receipt if received during the recipient’s normal business hours, or at the beginning of the recipient’s next business day after receipt if not received during the recipient’s normal business hours. All notices to be sent to a Party pursuant to this Agreement shall be sent to or made to the attention of such Party at the address set forth below or at such other address as such Party may stipulate to the other Party in the manner provided in this Section 8.

5205 N. O’Connor Blvd., Suite 200
Irving, Texas 75039
Phone: (972) 444-9001
Fax: (972) 969-3587
Attention: General Counsel

     9.  Entire Agreement . This Agreement and the Assignment and Assumption Agreement constitute the entire agreement of the Parties relating to the matters contained herein and therein, superseding all prior contracts or agreements, whether oral or