Background: Specialized 12-step based groups have emerged to address the needs of persons recovering from both substance abuse and psychiatric illness. Objective: This study investigates the role of social support in mediating the association between mutual aid participation and subsequent substance use for dually diagnosed persons. Method: A cohort of Double Trouble in Recovery (DTR) members in New York City were studied prospectively over a two-year period. Findings: Longer DTR participation during the first year of the study was associated with lower substance use in the second year; that effect was partially explained by the maintenance of high level of social support. Conclusion: These findings speak of the enduring influence of 12-step attendance on reducing substance use, and underline the importance of both 12-step attendance and supportive networks for dually diagnosed persons.