**Exhibit 31.1****CERTIFICATION**I, Jennifer L. Good, in my capacity as chief executive officer of Penwest Pharmaceuticals Co.,certify that: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 factor omit to state a material fact necessary to make the statements made, in light of thecircumstances under which such statements were made, not misleading with respect to the periodcovered by this report;3. Based on my knowledge, the financial statements, and other financial information included inthis report, fairly present in all material respects the financial condition, results ofoperations and cash flows of the registrant as of, and for, the periods presented in thisreport;4. I am responsible for establishing and maintaining disclosure controls and procedures (asdefined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal control over financialreporting (as defined in Exchange Act Rules 13a-15(f) and 15d-15(f)) for the registrant andhave: (a) Designed such disclosure controls and procedures, or caused such disclosure controlsand procedures to be designed under my supervision, to ensure that material informationrelating to the registrant, including its consolidated subsidiaries, is made known to me byothers within those entities, particularly during the period in which this report is beingprepared;  (b) Designed such internal control over financial reporting, or caused such internalcontrol over financial reporting to be designed under my supervision, to provide reasonableassurance regarding the reliability of financial reporting and the preparation of financialstatements for external purposes in accordance with generally accepted accountingprinciples;  (c) Evaluated the effectiveness of the registrant’s disclosure controls and procedures andpresented in this report my conclusions about the effectiveness of the disclosure controlsand procedures, as of the end of the period covered by this report based on suchevaluation; and  (d) Disclosed in this report any change in the registrant’s internal control over financialreporting that occurred during the registrant’s most recent fiscal quarter (theregistrant’s fourth fiscal quarter in the case of an annual report) that has materiallyaffected, or is reasonably likely to materially affect, the registrant’s internal controlover financial reporting; and5. I have disclosed, based on my most recent evaluation of internal control over financialreporting, to the registrant’s auditors and the audit committee of the registrant’s board ofdirectors (or persons performing the equivalent functions): (a) All significant deficiencies and material weaknesses in the design or operation ofinternal control over financial reporting which are reasonably likely to adversely affectthe 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 whohave a significant role in the registrant’s internal control over financial reporting.        Date: August 7, 2006 /s/ Jennifer L. Good   Jennifer L. Good   President and Chief Executive Officer