OBJECTIVE: This study examines barriers to participation and retention in 2 modalities (web-based and in-person) of a weight-management intervention tailored for individuals with serious mental illness (SMI). METHOD: Using a mixed-methods approach, we explored the barriers veterans with SMI face when participating in a web-based (WebMOVE) or in-person (MOVE-SMI) version of the same SMI-adapted MOVE weight-management program. Participants in the randomized controlled trial (n = 277) were recruited from specialty mental health clinics at a Veterans Affairs medical center. Barriers were analyzed across treatment condition and program attendance (engagement) at baseline and follow-up using a generalized lineal model. Post hoc analyses assessed whether changes in the trajectory of barriers over time were associated with engagement. A subsample of participants (n = 48) from the WebMOVE and MOVE-SMI treatment conditions completed a qualitative interview, and 2 coders used open coding to analyze the data. RESULTS: Although barriers specific to treatment modality existed, most barriers cut across intervention modality, including financial hardship, lack of reliable housing and transportation, comorbid physical and mental health issues, and competing demands on personal time. Results of post hoc analyses found the association between engagement and emotional and motivational factors to be statistically significant. CONCLUSIONS: This study is the 1st to identify barriers in a web-based intervention for SMI. Similar barriers persisted across treatment modalities. Known barriers, particularly socioeconomic barriers, should be addressed to improve engagement and retention of individuals in weight-management interventions adapted for SMI, irrespective of modality. (PsycINFO Database Record (c) 2019 APA, all rights reserved).