Little longitudinal research exists on the relationship between exercise self-determination and stage of change. This study investigated how self-determined motivation changes in patients with type 2 diabetes (N = 175) as they moved through the stages of change over a six-month exercise trial. Hierarchical linear modelling revealed that patients who progressed through the stages of exercise change had an overall increase in self-determined motivation, while non-progressors experienced a reduction in self-determined motivation from three to six months. These results indicate that individuals engaging in regular exercise at six months maintain initial increases in self-determined motivation. Findings are discussed in light of self-determination theory.