Though there is a high comorbidity between serious mental illness and substance use (dual diagnosis), little research has been conducted concerning the treatment of seriously mentally ill substance users. This study examined subjects (N=154) who participated in dual diagnosis treatment groups in a suburban out-patient community mental health center during a 10 year period, in order to determine the characteristics of subjects who responded well to outpatient group treatment. Data on 28 demographic, diagnostic, treatment and treatment participation variables were collected on each subject and the data was then analyzed by correlational, discriminant and multiple regression statistical procedures. Each of the procedures found that a more positive recovery status at admission combined with longer-term treatment and participation in additional treatment services were highly related to positive treatment outcomes. Four variables (more positive recovery status at admission, participation in a greater number of treatment groups, greater involvement in self-help groups and hospitalization while in treatment) were found significantly related to positive treatment outcome in all three statistical tests. There were also differences in gender and race/culture sub-groups concerning which variables contributed to positive treatment outcomes. / Master of Science