This study examined cognitive mediators of symptom change during exposure and response prevention (EX/RP) for obsessive–compulsive disorder (OCD). Based on cognitive models of OCD, obsessive beliefs were hypothesized as a mediator of symptom change. Participants were 70 patients with primary OCD receiving EX/RP either as part of a randomized controlled trial (n = 38) or in open treatment following nonresponse to risperidone or placebo in the same trial (n = 32). Blinded evaluations of OCD severity and self-report assessments of three domains of obsessive beliefs (i.e., responsibility/threat of harm, importance/control of thoughts, and perfectionism/intolerance of uncertainty) were administered during acute (Weeks 0, 4 and 8) and maintenance treatment (Weeks 12 and 24). Study hypotheses were examined using cross-lagged multilevel modeling. Contrary to predictions, the obsessive beliefs domains investigated did not mediate subsequent OCD symptom reduction. In addition, OCD symptoms did not significantly mediate subsequent change in obsessive beliefs. The present study did not find evidence of cognitive mediation during EX/RP for OCD, highlighting the need to investigate other plausible mediators of symptom improvement.