Cognitive-behavioral therapy (CBT) for bulimia nervosa (BN) has received considerable empirical support for its efficacy. However, few investigators have examined the mechanisms proposed to account for the reduction of BN symptoms during CBT. The current study examined the associations between therapist interventions, client mechanisms, and symptoms during treatment in a sample of 56 clients undergoing CBT for BN. Results suggested that behavioral interventions were most associated with symptom change during treatment whereas relational interventions were most associated with change in client mechanisms such as client engagement. Additionally, some changes in BN symptoms were mediated by changes in proposed client mechanisms whereas others were directly associated with therapist interventions. Implications of these findings for CBT theory of BN treatment and CBT treatment process research methodology are discussed.