While participatory evaluation (PE) constitutes an important trend in the field of evaluation, its ontology has not been systematically analyzed. As a result, the concept of PE is ambiguous and inadequately theorized. Furthermore, no existing instrument accurately measures stakeholder participation. First, this article attempts to overcome these problems by using the works of G. Goertz (2006) and J. Gerring (1999) on concept formation and evaluation to assess current conceptualizations of PE. Second, an amended version of the framework developed by J. B. Cousins and E. Whitmore (1998) is proposed as an alternative to current conceptualizations. This amended framework is then operationalized and adapted in a participation measurement instrument. The proposed conceptualization and instrument have the potential to contribute to the production of sound empirical knowledge about evaluation and to reflections on PE practice. (Contains 1 figure, 4 tables and 2 notes.)