While exposure and response prevention (ERP) is currently the most effective psychological treatment for unwanted, intrusive thoughts associated with Obsessive Compulsive Disorder (OCD), the procedures involved in ERP are challenging and may contribute to treatment refusal. To address this problem, researchers have begun to evaluate alternative treatments for OCD, such as Acceptance and Commitment Therapy (ACT). The purpose of the present study was to examine the relative effects of a single session of ACT or ERP for obsessional thoughts. Fifty-six undergraduate participants with obsessional thoughts were randomly assigned to receive a brief version of ERP, ACT, or a control condition. We found that the ERP group displayed greater willingness and that the ACT group demonstrated lower dysfunctional cognitions at follow-up. There were no differences between ACT and ERP on any other variables. Thus, no clear patterns emerged that suggest substantial differences between ACT and ERP in efficacy or change processes.