Exhibit 31
 CERTIFICATION
  1.
    
    I have reviewed this Quarterly Report on Form 10-Q 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
 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: August 10, 2009
    /s/ Jennifer L. GoodJennifer L. GoodPresident and Chief Executive Officer
(Principal Executive Officer and Principal
Financial Officer)