Assertive Community Treatment (ACT) is a community-based, multidisciplinary mental health treatment model with improved housing stability as a treatment goal. We know little about factors contributing to housing stability among ACT participants with co-occurring serious mental illness and substance use disorders, who account for 30% of the ACT participant population. Informed by the behavioral model of health service use, the present study aimed to examine the relationship between housing stability and theoretically relevant factors. We retrospectively abstracted the data from two ACT teams' treatment service planning and tracking system. Stable housing was defined by living in a private residence or permanent supportive housing throughout the assessment periods; unstable housing was defined by having at least one unstable housing situation (e.g., jail or prison) throughout the assessment periods. The sample included 57 individuals with 272 assessment charts. Multivariate logistic regression results show that service engagement was associated with housing stability and that receiving housing supportive services was inversely associated with housing stability. The findings support prior literature in that service engagement remained a "critical ingredient" of the ACT model and highlights the importance of the supportive aspect of housing services in improving housing stability among individuals with co-occurring disorders.