BACKGROUND: Most health behavior models do not distinguish between determinants of behavior adoption and maintenance. PURPOSE: This study compared psychosocial predictors of physical activity (PA) adoption and predictors of PA maintenance among 205 initially sedentary adults enrolled in a home-based PA promotion trial. METHODS: Psychosocial variables were measured at 6 months (at which point 107 participants remained inactive and 98 participants adopted regular PA) and used to predict 12-month PA status (an indicator of PA adoption among those inactive at 6 months and an indicator of PA maintenance among those active at 6 months). RESULTS: Six-month PA status moderated the relationships between 6-month measures of home access to PA equipment (p = .049), self-efficacy (p = .086), and perceived satisfaction (p = .062) and 12-month PA status. Simple effects analyses revealed that home access to PA equipment was predictive of PA adoption (OR = 1.73; 95% CI: 1.05, 2.85), but not PA maintenance (OR = 0.88; 95% CI: 0.58, 1.35), whereas self-efficacy and perceived satisfaction were predictive of PA maintenance (OR = 2.65; 95% CI: 1.55, 4.52; OR = 1.95; 95% CI: 0.93, 4.06), but not PA adoption (OR = 1.50; 95% CI: 0.87, 2.57; OR = 0.82, CI: 0.44, 1.52). CONCLUSION: Results suggest that these psychosocial variables may operate differently in predicting PA adoption versus maintenance.