Client competence consists of skill acquisition, comprehension, and mastery (Barber and DeRubeis 2001). Competence in the context of Cognitive Behavioral Therapy (CBT) specifically, could be conceptualized as an independent understanding of connections of behavior to thoughts and emotions, accepting a new cognitive or behavioral technique, demonstrating an ability to test the validity of negative cognitive content, and making behavioral changes such as decreasing avoidance. Several studies have shown that client competence is a predictor of treatment outcome (Barber and DeRubeis 2001; Jarrett, Vittengl, Clark, & Thase, 2011; Schmidt & Woolaway-Bickel, 2000; Strunk et al. 2007; Strunk et al., 2014). However, less is known of the specific predictors of competence. In the present study, a mediation model in which treatment engagement predicts treatment outcome through the mechanism of client competence, was tested in the context of group CBT for social anxiety disorder. The overall mediation model was not significant. However, medium effects of treatment engagement on competence and competence on social anxiety-related avoidance indicate that these variables contribute a small, yet meaningful percent of variance in overall outcome. Additionally, competence significantly predicted change in anxiety-related cognitions. Further research is needed to elucidate the processes that augment competence.