This study examined the effectiveness of training community mental health therapists in motivational interviewing (MI) adapted to treat clients with co-occurring disorders. Ten therapists with high caseloads of culturally diverse clients in two different community mental health settings fulfilled all study requirements. MI training consisted of a two-day didactic and experiential workshop followed by eight biweekly small group supervision (coaching) sessions. Using an interrupted time series design, 156 randomly selected therapy sessions involving 28 clients were coded for assessment of therapist fidelity to MI at multiple points in time, both pre- and post-training. Employing hierarchical linear modeling analysis, significant improvement in MI skill was observed after training on five of six key therapist ratings, and on the sole client rating (client change talk) that was examined. Importantly, the present study demonstrates training-related proficiency in motivational interviewing using: (a) a representative sample of mental health therapists from the community; (b) a protocol emphasizing adherence to a mental health treatment regimen as well as management of substance use behavior for clients with co-occurring disorders; (c) repeated random observations of therapy sessions; (d) measurement of training-related changes in clinician skills and self motivational statements by clients. Findings of this effectiveness study compared favorably with efficacy literature on MI training.