The Dual Diagnosis Capability in Mental Health Treatment (DDCMHT) Index was developed to assess the capability of mental health programs to provide substance abuse and co-occurring treatment services. The DDCMHT is an objective scale rated following a site visit that includes semi-structured interviews with staff at all levels, review of program documents and client charts, and ethnographic observation of the milieu and setting. Using data from 67 mental health programs across six states, this study found that the DDCMHT had excellent total score reliability, variable subscale reliability, high inter-rater reliability (n = 18), and moderate construct validity (n = 22). Results also suggest that many mental health programs are at a relatively low level of capability for the delivery of care to individuals with co-occurring disorders. Results from this important new benchmark measure, the DDCMHT, can be used with programs in implementation planning and with treatment systems, states, or national organizations to guide policy change.