In the context of a 6-month randomized controlled trial with an 18-month follow-up, we tested the extent to which adherence during the trial, affective responses to exercise, exercise value, and social support from the exercise group had either a direct or indirect effect through the mediation of exercise self-efficacy on physical activity levels at 6- and 18-month follow-up in a sample (N = 174) of older adults (M age, 66 years). Structural equation modeling tested several models of exercise prediction.