Based on the theory that generalized anxiety disorder (GAD) is maintained through a reactive and fused relationship with one’s internal experiences and a tendency towards experiential avoidance and behavioral restriction, an acceptance based behavioral therapy (ABBT) was developed to specifically target these elements. Since ABBT has been shown to be an efficacious treatment in previous studies, the current study focuses on proposed mechanisms of change over the course of therapy. Specifically, the current study focuses on session-by-session changes in two proposed mechanisms of change: acceptance of internal experiences and engagement in meaningful activities. Overall, clients receiving ABBT reported an increase in the amount of time spent accepting internal experiences and engaging in valued activities. Change in both acceptance and engagement in meaningful activities was related to responder status at post-treatment and change in these two proposed mechanisms predicted outcome above and beyond change in worry. In addition, change in acceptance was related to reported quality of life at post-treatment.