Despite evidence supporting a robust relation between common factors- aspects of the therapeutic setting that are common across all types of treatment- and treatment response, little is known about the mechanisms by which these common factors effect change. Two of the most well-researched common factors include the client’s expectations about the effectiveness of treatment (termed outcome expectancy), and the quality of the therapeutic relationship (termed working alliance). Using archival data, the present study tests the hypothesis that the relation between outcome expectancy and symptom reduction is mediated by the alliance following treatment for social anxiety disorder. Data were collected in a sample of 65 individuals who received cognitive behavioral therapy for social phobia with public speaking fears. Mediation analyses were conducted using Andrew Hayes’ Process Macro (Hayes, 2013). None of the mediation analyses were significant. These findings suggest that the mechanisms of common factor variables may vary by disorder.