**EXHIBIT 31.1**

**CERTIFICATION OF PRINCIPAL EXECUTIVE OFFICER******Section 302 Certification

I, John R. Gibson, certify that:

|   | 1.     | I have reviewed this annual report on Form 10-K of   American Pacific Corporation, a Delaware corporation (the “registrant’’);                                                                                                                                                                               |
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|   | 2.     | Based on my knowledge, this report does not contain   any untrue statement of a material fact or omit to state a material fact   necessary in order to make the statements made, in light of the circumstances   under which such statements were made, not misleading with respect to the   period covered by this report; |
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|   | 3.     | Based on my knowledge, the financial statements, and   other financial information included in this report, fairly present in all   material respects the financial condition, results of operations and cash   flows of the registrant as of, and for, the periods presented in this report;                               |
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|   | 4.     | The registrant’s other certifying officer and I are   responsible for establishing and maintaining disclosure controls and   procedures (as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) for the   registrant and have:                                                                                      |
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|   |   | (a)                                                                                                                                                                                                                                                                                                                         | Designed such disclosure controls and procedures, or   caused such disclosure controls and procedures to be designed under our   supervision, to ensure that material information relating to the registrant,   including its consolidated subsidiaries, is made known to us by others within   those entities, particularly during the period in which this report is being   prepared; |
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|   |   | (b)                                                                                                                                                                                                                                                                                                                         | Evaluated the effectiveness of the registrant’s   disclosure controls and procedures and presented in this report our conclusions   about the effectiveness of the disclosure controls and procedures, as of the   end of the period covered by this report based on such evaluation; and |
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|   |   | (c)                                                                                                                                                                                                                                                                                                                         | Disclosed in this report any change in the   registrant’s internal control over financial reporting that occurred during   the registrant’s most recent fiscal quarter (the registrant’s fourth fiscal   quarter in the case of an annual report) that has materially affected, or is   reasonably likely to materially affect, the registrant’s internal control over   financial reporting; and |
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|   | 5.     | The registrant’s other certifying officer and I have   disclosed, based on our most recent evaluation of internal control over   financial reporting, to the registrant’s auditors and the audit committee of   registrant’s board of directors (or persons performing the equivalent   function):           |
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|   |   | (a)                                                                                                                                                                                                                                                                                                                         | All significant deficiencies and material weaknesses   in the design or operation of internal control over financial reporting which   are reasonably likely to adversely effect the registrant’s ability to record,   process, summarize and report financial information; and |
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|   |   | (b)                                                                                                                                                                                                                                                                                                                         | Any fraud, whether or not material, that involves   management or other employees who have a significant role in the registrant’s   internal control over financial reporting. |

 

| Date:     December 18, 2003 |   | By:    | /s/ JOHN R. GIBSON                                  |   |
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|                                                 |   |   | **John R.   Gibson,   Principal Executive Officer** |   |