Internet-based cognitive behavior therapy (ICBT) for obsessive-compulsive disorder (OCD) has shown efficacy in randomized trials but many patients do not respond to the treatment, we therefore need to find predictors and moderators of treatment response. In this study, we analyzed predictors of ICBT response using both post-treatment as well as 24-month outcome data. As half of the participants were randomized to receive an Internet-based booster program as an adjunct to ICBT, we also investigated moderators of ICBT with or without booster. Results showed that more severe baseline OCD symptoms predicted worse end state outcome but also higher degree of change. Furthermore, high degree of working alliance predicted better outcome but patients with primary disgust emotions had worse treatment effects. The moderator analysis also indicated that scoring high on the obsessing subscale on the Obsessive-Compulsive Inventory-Revised predicted worse treatment outcome in the booster group. In conclusion, there are some possible predictors and moderators of ICBT for OCD but more research is needed with larger and clinically representative samples. (C) 2014 Elsevier Inc. All rights reserved.