Pioneer to Offer 8.25 Million Common Units

Pioneer Natural Resources Company announced that Pioneer Southwest Energy Partners L.P. has commenced an initial public offering of 8,250,000 common units representing limited partner interests pursuant to a registration statement on Form S-1 previously filed with the Securities and Exchange Commission. The underwriters will be granted a 30-day option to purchase up to an additional 1,237,500 common units to cover over-allotments. The Pioneer Southwest Energy common units will be listed on the New York Stock Exchange and traded under the symbol "PSE."

The common units offered to the public will represent approximately 28.7% of the limited partner interest of Pioneer Southwest Energy, or approximately 31.6% if the underwriters exercise their over-allotment option in full. Pioneer will own a 0.1% general partner interest and a 71.3% limited partner interest in Pioneer Southwest Energy. Pioneer's limited partner interest would be reduced to 68.4% if the underwriters exercise their over-allotment option in full.

Pioneer Southwest Energy is a Delaware limited partnership formed by Pioneer to own producing oil and gas properties in the Spraberry field in the Permian Basin of West Texas and to acquire oil and gas assets in its area of operations. This area includes onshore Texas and eight counties in the southeast region of New Mexico.

Citigroup Global Markets Inc., Deutsche Bank Securities Inc. and UBS Securities LLC will act as joint book-running managers for the offering.

This offering of common units will be made only by means of a prospectus. A copy of the prospectus may be obtained by submitting requests to Citigroup Global Markets Inc., Attention: Prospectus Department, Brooklyn Army Terminal, 140 58th Street, 8th Floor, Brooklyn, New York 11220, phone: 718-765-6732, fax: 718-765-6734; Deutsche Bank Securities Inc., Attn: Prospectus Department, 100 Plaza One, 2nd Floor, Jersey City, NJ, 07311, or by telephone at (800) 503-4611, or by email at; or UBS Securities LLC, Attention: Prospectus Department, 299 Park Avenue, New York, New York 10171, phone: 212-821-3000.

A registration statement relating to these securities has been filed with the Securities and Exchange Commission but has not yet become effective. These securities may not be sold, nor may offers to buy be accepted, prior to the time the registration statement becomes effective. This communication does not constitute an offer to sell or the solicitation of an offer to buy nor shall there be any sale of these securities in any state in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such state or jurisdiction.