**EXHIBIT 31.1**

**CERTIFICATION OF PRINCIPAL EXECUTIVE OFFICER**

Section 302 Certification

I, John R. Gibson, certify that:

| 1)     | I have reviewed this quarterly report on Form 10-Q   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 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(s) 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(s) 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   the registrant’s board of directors (or persons performing the equivalent   functions): |
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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 affect 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: February 4, 2004 | /s/  JOHN R. GIBSON                   |
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|                   | **John R.   Gibson    Chief Executive Officer** |