<PAGE>


                                                                      EXHIBIT 31

                                 CERTIFICATIONS

I, Jennifer L. Good, in my capacity as principal executive officer and principal
financial officer of Penwest Pharmaceuticals Co., certify that:

1.   I have reviewed this Annual Report on Form 10-K of Penwest Pharmaceuticals
     Co.;

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;

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;

4.   I am responsible for establishing and maintaining disclosure controls and
     procedures (as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and
     internal control over financial reporting (as defined in Exchange Act Rules
     13a-15(f) and 15d-15(f)) for the registrant and have:

     (a)  Designed such disclosure controls and procedures, or caused such
          disclosure controls and procedures to be designed under my
          supervision, to ensure that material information relating to the
          registrant, including its consolidated subsidiaries, is made known to
          me by others within those entities, particularly during the period in
          which this report is being prepared;

     (b)  Designed such internal control over financial reporting, or caused
          such internal control over financial reporting to be designed under my
          supervision, to provide reasonable assurance regarding the reliability
          of financial reporting and the preparation of financial statements for
          external purposes in accordance with generally accepted accounting
          principles;

     (c)  Evaluated the effectiveness of the registrant's disclosure controls
          and procedures and presented in this report my 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

     (d)  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

<PAGE>

5. I have disclosed, based on my 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.


Date: March 13, 2006                       /s/ Jennifer L. Good
                                           -------------------------------------
                                           Jennifer L. Good
                                           President and Chief Operating Officer
                                           (principal executive officer and
                                           principal financial officer)