BackgroundImproving the delivery of cognitive-behavioural therapy (CBT) for social anxiety disorder (SAD) requires an in-depth understanding of which cognitive and behavioural mechanisms drive change in social anxiety symptoms (i.e., social interaction anxiety) during and after treatment. The current study explores the dynamic temporal associations between theory-driven cognitive and behavioural mechanisms of symptom change both during and following group CBT. MethodsA randomized controlled trial of imagery-enhanced CBT (n = 51) versus traditional verbal CBT (n = 54) for social anxiety was completed in a community mental health clinic setting. This study included data collected from 12-weekly sessions and a 1-month follow-up session. Mixed models were used to assess magnitude of change over the course of treatment. Cross-lagged panel models were fit to the data to examine temporal relationships between mechanisms (social-evaluative beliefs, safety behaviours) and social interaction anxiety symptoms. ResultsParticipants in both CBT groups experienced significant improvements across all cognitive, behavioural, and symptom measures, with no significant differences in the magnitude of changes between treatments. During treatment, greater social-evaluative beliefs (fear of negative evaluation, negative self-portrayals) at one time point (T) were predictive of more severe SAD symptoms and safety behaviours at T+1. Social-evaluative beliefs (fear of negative evaluation, probability and cost of social failure) and safety behaviours measured at post-treatment were positively associated with SAD symptoms at the 1-month follow-up. ConclusionsThe current study identifies social-evaluative beliefs that may be important targets for symptom and avoidance reduction during and following CBT. Assessment of these social-evaluative beliefs throughout treatment may be useful for predicting future SAD symptoms and avoidance, and for adapting treatment to promote optimal change for patients.