Abstract: Substance dependence and treatment have been the focus of research for a number of years. Recently more attention has been directed toward identifying the most appropriate treatments for people with more complicated substance disorders, those with substance dependence and mental illness. The current study examined three aspects of group cohesion, a necessary condition for effective outcomes, in 64 substance abusers with a comorbid mental illness. The rated aspects of group cohesion included overall group climate, individual group members' self-worth, and each member's perception of the relationship with the group leader. Groups were divided according to profiles based on results from the Millon Clinical Multiaxial Inventory-III. Groups included Cluster A personality disorders, Cluster B personality disorders, Cluster C personality disorders, and patients having no personality disorder. Group climate was rated using the Group Climate Questionnaire, self-worth was measured using the Contingencies of Self-Worth, and the therapeutic alliance was measured using the Working Alliance Inventory - Short Form. Results indicated significant differences between the four groups in overall ratings of the therapeutic alliance and in the aspect of group climate identified as conflict. These differences indicate the need to investigate these process variables further and may result in more effective group therapy.