Little research has been conducted on the theoretical mechanisms of exercise post cardiac rehabilitation (CR). The Theoretical Domains Framework (TDF), as embedded within the Capability, Opportunity, Motivation, and Behaviour model (COM-B), can provide a theoretical understanding of exercise post-CR. This study aimed at identifying TDF-related psychosocial and environmental variables that were important for predicting moderate to vigorous physical activity (MVPA) in individuals post-CR. Eighty-four individuals (80% men; Mage=60 years, SD=15.83) who completed a CR program answered baseline questionnaires assessing ten TDF domains. Participants wore an accelerometer for 7 days three months after baseline assessments, from total minutes of were computed. Six multiple regressions were conducted to predict MVPA at three months. TDF domains for each COM-B sub-component (e.g., psychological capability) were tested in five separate regressions. A final regression included significant variables from the five regressions to determine the strongest TDF predictor. Decisional balance (? = .28) as part of Capability, streets, walking/cycling, and neighbourhood surroundings (? = .20) for the Opportunity component, negative affect (? = -.09) and identity (? = -.14) within Motivation were significant predictors of MVPA in separate regression models. The final regression explained 26% of the variance in MVPA at three months and intention was the sole significant predictor (? = .33, p< 0.05). Thus, it is important to promote participants' intentions to continue exercise post-CR to help sustained physical activity participation.