Relatively little is known about how substance abuse treatment facilitates positive outcomes. This study examined the therapeutic effects and mechanisms of action of affiliation with Alcoholics Anonymous (AA) after treatment. Patients (N = 100) in intensive 12-step substance abuse treatment were assessed during treatment and at 1- and 6-month follow-ups. Results indicated that increased affiliation with AA predicted better outcomes. The effects of AA affiliation were mediated by a set of common change factors. Affiliation with AA after treatment was related to maintenance of self-efficacy and motivation, as well as to increased active coping efforts. These processes, in turn, were significant predictors of outcome. Findings help to illustrate the value of embedding a test of explanatory models in an evaluation study.