The threshold of change model (TCM) is a statistical technique for analyzing ordered stages of change variables. TCM focuses on the thresholds that separate the ordered stages, and the effects of explanatory variables are evaluated in terms of raising or lowering the thresholds. TCM also allows the explanatory variables to exert differential influence on each threshold. In this paper, the authors use TCM to analyze the data from a clinical trial that compared assertive community treatment (ACT) with standard case management (SCM) for patients with co-occurring severe mental illness and substance use disorder. Endpoint data (36-mo follow-up) were used for this analysis. The response variable is the recoded Substance Abuse Treatment Scale with 3 ordered levels (engagement/persuasion, active treatment, and recovery/relapse prevention), and hence 2 thresholds. The explanatory variables are gender and group (ACT vs SCM). The results indicate that gender exerts constant and significant effects on both thresholds. The group effect is somewhat mixed: ACT lowers the 1st threshold (active treatment), but raises the 2nd threshold (recovery/relapse prevention).