This study describes the development and validation of the Evaluation Capacity Assessment Instrument (ECAI), a measure designed to assess evaluation capacity among staff of nonprofit organizations that is based on a synthesis model of evaluation capacity. One hundred and sixty-nine staff of nonprofit organizations completed the ECAI. The 68-item measure assessed participants' perceptions of individual and organizational predictors of two evaluation capacity outcomes: mainstreaming and use of evaluation findings. Confirmatory Factor Analysis and internal consistency results support the inclusion of the items and factors measured by the ECAI. Moreover, structural equation modeling results support the synthesis model and its depiction of relationships among evaluation capacity predictors and outcomes. We discuss the implications of using a validated model and instrument in evaluation capacity building research and practice. (Contains 4 tables, 1 figure and 1 note.)