Little is known about how outcome expectations change after physical activity initiation and whether changes are associated with physical activity experiences. In a diary study, physically inactive adults (N = 102) initiated an exercise regimen and reported their experiences daily (e.g. progress toward goals) and corresponding outcome expectations weekly (e.g. how much progress they expect this week). Average levels (between-person effects) for eight experiences (ps < .01) and deviations from the average levels (within-person effects) for three experiences (ps < .05) were associated with changes in outcome expectations. The findings demonstrate that outcome expectations for exercise vary over time and are associated with people’s subjective experiences.