Behavioral science theories have been used to develop physical activity interventions; however, little is known as to whether these interventions are effective due to changes in constructs related to these theories. Specifically, if the intervention is successful, does it work for the reasons hypothesized by the theory underlying it? The purpose of this study was to examine the importance of particular theoretical constructs among participants (n = 150) who had been randomly assigned to a physical activity intervention based on the Transtheoretical Model and Social Cognitive Theory (i.e., tailored group) or to a standard care group. Participants in the tailored group reported greater increases in behavioral processes and self-efficacy from baseline to 3 months than participants in the standard-care group. No between-group differences were found for cognitive processes and decisional balance. This study demonstrates that theory-based physical activity interventions may be effective through changes in particular theoretical constructs.