BACKGROUND : We examined whether providing three sessions of treatment based on motivational interviewing (MI) prior to Cognitive Behavioral Therapy (CBT) for Social Anxiety Disorder (SAD) improved outcomes. METHODS : Participants diagnosed with SAD (N = 186) were randomly allocated to receive three sessions of MI (MI+CBT; n = 85) or supportive counselling (SC+CBT; n = 101) prior to a 12-week group CBT program. Assessments occurred at baseline, after preparatory treatment, after CBT, and at 6-months follow-up. Outcomes were expectations for change, number of CBT sessions attended, self- and clinician-rated CBT homework completion, and self- and clinician-rated social anxiety severity. RESULTS : Conditions did not differ significantly on expectations for change, number of CBT sessions attended, or clinician-rated homework completion. Self-rated homework completion was greater in MI+CBT than in SC+CBT. Change over time in social anxiety severity did not differ between conditions overall, however, this outcome was significantly moderated by two variables; those in MI+CBT, as compared to SC+CBT, showed significantly poorer outcomes on self-reported social anxiety severity if they were higher in change readiness and significantly better outcomes on clinician-rated social anxiety severity if they were higher in functional impairment. CONCLUSIONS : Although therapists in MI sessions were rated as behaving more consistently with MI than therapists in SC sessions, some MI consistent behaviors occurred in the SC sessions. CONCLUSIONS : Addition of a MI-based discussion prior to evidence-based CBT appears to benefit people with SAD who have high functional impairment but may interfere with outcomes for those higher in readiness for change.