Ampac-ISP Corp., a wholly-owned subsidiary of American Pacific Corporation (Nasdaq: APFC – News), reported that their LEROS 1b thruster successfully completed a 524-second burn for the Mercury MESSENGER spacecraft on December 12, 2005, putting the solar-powered spacecraft on track for its first scheduled flyby of Venus in October 2006. This maneuver, known as Deep Space Maneuver 1 (DSM 1) is the first firing of the LEROS 1b for this mission, which started in August 2004 with its launch aboard a Delta II rocket from Cape Canaveral. MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) is the seventh mission in NASA’s Discovery Program, and will study Mercury’s atmosphere, topography and elemental makeup. The engine will be fired at least five more times to place the spacecraft in orbit around Mercury. “We are proud to provide this key propulsion component as a member of the Mercury MESSENGER team,” said Robert Huebner, Vice President, AMPAC-ISP Operations. “The successful firing of the LEROS 1b underscores the reliability and effectiveness of our rocket engines.”

The LEROS 1b is a 645N class Hydrazine/NTO bipropellant thruster that was first qualified in 1995 and has been flying on telecommunications and military missions since 1996. The LEROS 1b was also used on the Mars Global Surveyor and Mars Odyssey spacecraft for major mission maneuvers.

The LEROS 1b was developed and is manufactured at AMPAC-ISP’s Westcott, United Kingdom division, with final testing at our Niagara Falls, New York facility. The LEROS 1b combines high thrust with high specific impulse (318 lbf-sec/lbm) to meet the control authority needs and provide thrust margin for heavy payloads and deep space probes. More than seventy (70) LEROS 1 series engines have been delivered since 1991 and all have performed flawlessly. A LEROS 1 engine provided the impulse needed for major orbital maneuvers during the mission to the asteroid Eros for the NEAR Mission.

About Ampac-ISP Corp.

Ampac-ISP Corp. is a leader in the development and manufacturing of liquid rocket engines, tanks and propulsion systems for commercial and military launch vehicles and space systems. Ampac-ISP, with facilities in Niagara Falls, New York and Westcott, United Kingdom, is a wholly-owned subsidiary of American Pacific Corporation.

About American Pacific Corporation

American Pacific is a chemical and aerospace company with the following products: (i) fine chemicals in the form of active pharmaceutical ingredients and registered intermediates, (ii) perchlorate chemicals used in space propulsion and other applications, (iii) liquid in-space propellant thrusters used for attitude control on satellites, (iv) Halotron, a clean fire extinguishing agent, (v) sodium azide and other energetic products used in various applications and (vi) water treatment equipment. Fine chemicals are produced at Ampac Fine Chemicals (AFC) in Rancho Cordova, California. Perchlorates and other energetic chemicals as well as Halotron and water treatment equipment (PSI) are manufactured at the Wecco division in Cedar City, Utah. The in-space propulsion business, Ampac-ISP, is located at Niagara Falls, New York, and Westcott in the United Kingdom. American Pacific also participates in a joint venture packaged explosive manufacturer, Energetic Systems, Inc. (ESI) with facilities in Hallowell, Kansas and Oklahoma City, Oklahoma. Additional information about American Pacific can be obtained by visiting the Company’s web site at www.apfc.com.

Risk Factors/Forward Looking Statements

The statements contained in this press release that are not purely historical are forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995, Section 27A of the Securities Act of 1933 and Section 21E of the Securities Act of 1934. Such forward-looking statements involve known and unknown risks and uncertainties that could cause actual results of the Company, or industry results, to differ materially from any future results, performance, or achievements expressed or implied by such forward-looking statements. Readers of this release are referred to the American Pacific Corporation Annual Report on Form 10-K for the year ended September 30, 2005, and the American Pacific Corporation Quarterly Report on Form 10-Q for the quarter ended March 31, 2006, for further discussion of these and other factors that could affect future results. The forward-looking statements contained in this news release are made as of the date hereof, and both Ampac-ISP Corp. and American Pacific Corporation assume no obligation to update for actual results or to update the reasons why actual results could differ materially from those projected in the forward-looking statements.

Website: www.apfc.com