Objectives: A limitation of adult day service (ADS) research is that there remains little understanding of how these community-based long-term care programs operate to benefit clients or family caregivers (i.e. the process of ADS use). The purpose of this study was to validate the ‘ADS Process and Use Measures’ (APUM) which were developed to assess such mechanisms. Method: Participant observation and semi-structured interviews in two ADS settings resulted in qualitative data to inform a conceptual model, subscales, and Likert-scale items. Three experts in ADS research reviewed the initial 129-item version of the APUM to establish content validity, and 27 family caregivers of current or prior ADS clients provided feedback on face validity of a subsequent 58-item version. Results: Principal components and confirmatory factor analyses on a sample of 269 family members of ADS clients recruited from 90 programs throughout the U.S. established a measure featuring 5 domains, 12 reliable subscales, and 49 items. Analysis of discriminant and convergent validity found that various subscales from four of the domains (Why ADS is Used, Events Prior to Use, Why ADS Does Not Work, and Pathways to Benefits) were significantly associated (p < 0.05) with family caregiver distress and ADS client quality of life variables. Conclusion: The ADS Process and Use Measures effectively assess mechanisms of program benefit and could help to enhance the overall quality of these critical community-based long-term care options for older persons and their families.