<PAGE>

                                                                    Exhibit 31.1

                      CHIEF EXECUTIVE OFFICER CERTIFICATION


I, Leonard S. Schwartz, Chairman, President and Chief Executive Officer of Aceto
Corporation, certify that:


        1. I have reviewed this Quarterly Report on Form 10-Q of Aceto
        Corporation (the "Registrant");

        2. Based on my knowledge, this Quarterly 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 Quarterly Report;

        3. Based on my knowledge, the financial statements, and other financial
        information included in this Quarterly 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 Quarterly Report;

        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:

        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
        Quarterly Report is being prepared;

        b) evaluated the effectiveness of the Registrant's disclosure controls
        and procedures and presented in this Quarterly 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

        c) disclosed in this Quarterly Report any change in the Registrant's
        internal control over financial reporting that occurred during the
        Registrant's first fiscal quarter that has materially affected, or is
        reasonably likely to materially affect, the Registrant's internal
        control over financial reporting; and

        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 the
        Registrant's board of directors (or persons performing the equivalent
        functions):

        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

        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.

Dated: February 9, 2005

/s/ Leonard S. Schwartz

Chairman, President and Chief Executive Officer
(Principal Executive Officer)
--------------------------------------------------------------------------------