CONTRIBUTION AGREEMENT

EXHIBIT 2.1

EXECUTION VERSION

CONTRIBUTION AGREEMENT
by and among
PIONEER NATURAL RESOURCES USA, INC.
PIONEER NATURAL RESOURCES GP LLC
and
PIONEER SOUTHWEST ENERGY PARTNERS L.P.

May 6, 2008

TABLE OF CONTENTS

		Page
ARTICLE I DEFINITIONS
1.1 Definitions			2
1.2 Construction			3
ARTICLE II CONTRIBUTIONS
2.1 Contribution by Pioneer USA to the General Partner of the GP Contribution Interest in Operating Company			4
2.2 Contributions by Pioneer USA and the General Partner			4
2.3 Redemption of the Initial General Partner Interest and the Initial Limited Partner Interest			4
ARTICLE III MISCELLANEOUS
3.1 Order of Completion of Transactions			5
3.2 Investment Intent			5
3.3 No Representations and Warranties			5
3.4 Choice of Law			5
3.5 Notice			5
3.6 Entire Agreement			6
3.7 Jurisdiction; Service of Process			6
3.8 Further Action			6
3.9 Binding Effect			6
3.10 Effect of Waiver or Consent			6
3.11 Counterparts			6
3.12 Invalidity of Provisions			6
3.13 Assignment			7
3.14 Direct or Indirect Action			7
3.15 Laws and Regulations			7
3.16 No Recourse Against Officers, Directors, Managers or Employees			7
3.17 Arbitration			8

CONTRIBUTION AGREEMENT

THIS CONTRIBUTION AGREEMENT, dated as of May 6, 2008, is entered into by and among Pioneer Natural Resources USA, Inc., a Delaware corporation (“ Pioneer USA ”), Pioneer Natural Resources GP LLC, a Delaware limited liability company (the “ General Partner ”), and Pioneer Southwest Energy Partners L.P., a Delaware limited partnership (“ MLP ”). The foregoing parties hereto are referred to in this Agreement individually as a “Party” and collectively as the “Parties.” Each capitalized term used herein that is defined in Section 1.1 shall have the meaning assigned to such term in Section 1.1. Other terms defined herein have the meanings so given them.

RECITALS

     A. Pioneer USA and the General Partner have formed MLP pursuant to the Delaware Revised Uniform Limited Partnership Act (the “ Delaware Limited Partnership Act ”), for the purpose of engaging in any business activity that is approved by the General Partner, and that lawfully may be conducted by a limited partnership organized pursuant to the Delaware Limited Partnership Act.

     B. In order to accomplish the objectives and purposes in Recital A, the following actions have been taken prior to the date hereof:

	1.		Pioneer USA formed the General Partner under the terms of the Delaware Limited Liability Company Act and contributed $1,000 in exchange for all of the membership interest in the General Partner.
	2.		Pioneer USA and the General Partner formed MLP under the terms of the Delaware Limited Partnership Act to which the General Partner contributed $1 and Pioneer USA contributed $999 in exchange for a .1% general partner interest (the “ Initial General Partner Interest ”) and a 99.9% limited partner interest (the “ Initial Limited Partnership Interest ”) in MLP, respectively.
	3.		Pioneer USA formed Operating Company under the terms of the Texas Limited Liability Company Act and contributed $1,000 in exchange for all of the membership interests in Operating Company.
	4.		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 Membership Interest Sale 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.

NOW, THEREFORE, in consideration of their mutual undertakings and agreements hereunder, the Parties undertake and agree as follows:

ARTICLE I
DEFINITIONS

     1.1 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 ” means this Contribution Agreement.

          “ Assets ” means the membership interests in the Operating Company contributed to MLP pursuant to the terms of this Agreement.

          “ Conflicts Committee ” has the meaning assigned to such term in the Partnership Agreement.

          “ Delaware Limited Partnership Act ” has the meaning assigned to such term in Recital A to this Agreement.

          “ Dispute ” has the meaning assigned to such term in Section 3.17 to this Agreement.

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

          “ GP Contribution Interest ” means a 0.10295172491645% membership interest in the Operating Company.

          “ Initial General Partner Interest ” has the meaning assigned to such term in Item 2 of Recital B to this Agreement.

          “ Initial Limited Partner Interest ” has the meaning assigned to such term in Item 2 of Recital B to this Agreement.

          “ 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.

          “ Limited Partners ” has the meaning assigned to such term in the Partnership Agreement.

          “ Membership Interest Sale Agreement ” means the Membership Interest Sale Agreement dated May 6, 2008 between Pioneer USA and MLP, which agreement is being entered into simultaneously with the execution of this Agreement.

          “ 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 ” means Pioneer Southwest Energy Partners USA LLC, a Texas limited liability company.

          “ Partnership Agreement ” means the First Amended and Restated Agreement of Limited Partnership of MLP, dated as of May 6, 2008, as the same may be amended or restated from time to time.

          “ 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.

          “ Units ” means 20,521,200 common units representing limited partner interests in MLP.

          “ 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 partners interests in MLP.

          1.2 Construction . Unless the context requires otherwise: (1) any pronoun used in this Agreement shall include the corresponding masculine, feminine or neuter forms, and the singular form of nouns, pronouns and verbs shall include the plural and vice versa; (2) references to Articles and Sections refer to Articles and Sections of this Agreement; (3) the terms “include,”

“includes,” “including” and words of like import shall be deemed to be followed by the words “without limitation”; (4) the terms “hereof,” “herein” and “hereunder” refer to this Agreement as a whole and not to any particular provision of this Agreement; and (5) references to money refer to legal currency of the United States of America. The table of contents and headings contained in this Agreement are for reference purposes only, and shall not affect in any way the meaning or interpretation of this Agreement.

ARTICLE II
CONTRIBUTIONS

     2.1 Contribution by Pioneer USA to the General Partner of the GP Contribution Interest in Operating Company . Immediately after the issuance of the IPO Units, and prior to the transactions described in Section 2.2, Pioneer USA hereby grants, contributes, transfers, assigns and conveys to the General Partner, its successors and assigns, for its and their own use forever, all of Pioneer USA’s right, title and interest in and to the GP Contribution Interest, and the General Partner hereby accepts the GP Contribution Interest as an addi