UNLABELLED: Cognitive-Behavioral Group Therapy (CBGT) and Mindfulness-Based Stress Reduction (MBSR) are efficacious in treating social anxiety disorder (SAD). It is not yet clear, however, whether they share similar trajectories of change and underlying mechanisms in the context of SAD. This randomized controlled study of 108 unmedicated adults with generalized SAD investigated the impact of CBGT vs. MBSR on trajectories of social anxiety, cognitive reappraisal, and mindfulness during 12 weeks of treatment. CBGT and MBSR produced similar trajectories showing decreases in social anxiety and increases in reappraisal (changing the way of thinking) and mindfulness (mindful attitude). Compared to MBSR, CBGT produced greater increases in disputing anxious thoughts/feelings and reappraisal success. Compared to CBGT, MBSR produced greater acceptance of anxiety and acceptance success. Granger Causality analyses revealed that increases in weekly reappraisal and reappraisal success predicted subsequent decreases in weekly social anxiety during CBGT (but not MBSR), and that increases in weekly mindful attitude and disputing anxious thoughts/feelings predicted subsequent decreases in weekly social anxiety during MBSR (but not CBGT). This examination of temporal dynamics identified shared and distinct changes during CBGT and MBSR that both support and challenge current conceptualizations of these clinical interventions. CLINICALTRIALS. GOV IDENTIFIER: NCT02036658.