The primary purpose of this study was to determine if changes in physical self-efficacy, physical self-perceptions, and affect (including enjoyment) are mechanisms by which a single bout of exercise improves state body image. A secondary purpose was to identify how long exercise-related improvements in state body image are sustained post-exercise. Sixty women with body image dissatisfaction were randomized to either an exercise or control condition. Participants in the exercise condition completed a 30 minute moderate-to-vigorous intensity workout on a stationary bike. Women in the control condition read for the same duration. Before and immediately, 10-minutes and 20-minutes after (post-0, post-10, post-20) the exercise/control manipulations, participants completed measures of state body image, aerobic self-efficacy, physical self-perceptions and affect (including enjoyment). PROCESS macro was used to test mediation models, and a repeated measures mixed ANCOVA, followed by a univariate ANCOVA was used to test how long state body image improvements were sustained. There was a significant indirect effect of increases in strength self-perceptions on state body image improvements from pre- to post-0 (95% C.I. =.07 - .52, κ2=.16, abps=.31), post-10 (95% C.I.=.05 - .50, κ2=.16, abps=.30) and post-20 minutes (95% C.I. = .10 - .55, κ2 =.16, abps =.33), as well as a significant indirect effect of decreases in tiredness (i.e., increases in the energetic arousal dimension) on state body image improvements from pre- to post-10 (95% C.I. =.22 – 1.14, κ2=.30, abps=.77). Exercise-related state body image improvements were sustained 20 minutes after exercise. The present findings suggest that a bout of exercise improves state body image through increases in strength self-perceptions and energetic arousal and these effects lasted up to 20 minutes. This is the first study to design and empirically test a model to account for the effects of exercise on state body image, and provides important theoretical and practical implications. / Thesis / Master of Science (MSc)