Abstract:Background: Although internet-based cognitive behavior therapy (ICBT) is an effective treatment approach for social anxiety disorder (SAD), a substantial proportion of patients do not achieve clinically significant improvement. More research is needed to identify which factors predict treatment adherence and outcomes. Objective: The aims of this study were to (1) identify demographic and clinical factors that are associated with treatment adherence and outcomes in ICBT for social anxiety in China, (2) explore whether a low-intensity therapist support approach results in improved treatment adherence or outcomes and (3) adopt qualitative methods to explore motivating factors and barriers of adherence as well as other potentially enhancing strategies. Methods: Data were collected within the context of a previously published study, in which participants (N=255) were assigned to either therapist-guided ICBT (N=183) or self-guided ICBT (N=72) groups. The Chinese version of the Social Phobia Scale (SPS) and Social Interaction Anxiety Scale (SIAS) were completed at both pre-and post-treatment. Treatment adherence and outcomes were analyzed using a two-step linear and logistic regression approach. Clinical and demographic characteristics were examined. Qualitative data concerning motivating factors and barriers to adhere to ICBT were obtained. Results: No significant group differences were found for treatment adherence or outcomes (Ps > . 05). Participants diagnosed with Social Anxiety Disorder were significantly less likely to dropout (OR 0.531, P = .03). Furthermore, older participants (B = 0.17, SE = 0.04, P = .008) and participants with a diagnosis of SAD (B = 0.16, SE = 0.44, P = .01) tended to complete more modules. Participants who completed more modules (B = 0.24, SE = 0.03, P = .01) and participants who identified as female (B = -0.20, SE = 0.18, P = .04) reported greater improvements on the SIAS. Conclusions: Motivating factors and barriers to adherence, clinical implications, limitations, and future directions are discussed. Several recommendations are proposed for future research: (1) identify and evaluate more effective forms of low-intensity therapist support for enhancing treatment outcomes and adherence in ICBT for SAD; (2) include larger, more diverse samples; and (3) develop a coherent theoretical model of treatment adherence in online interventions.