Checking behavior (CB) occurs in a variety of disorders such as obsessive-compulsive (OCD), body dysmorphic (BDD), illness anxiety (IA), and panic disorder (PD), as well as anorexia (AN) and bulimia nervosa (BN). Etiological models of these disorders - with the exception of those for PD - postulate that CB mainly occurs in situations characterized by negative affect and serves to regulate it. We aimed to test these assumptions: N = 386 individuals with a self-reported diagnosis of one of the disorders rated their affect at baseline, directly before a remembered CB episode, during, immediately afterwards, and 15 and 60 minutes afterwards, and rated their endorsement of different functions of CB. The results show that transdiagnostically negative affect is significantly higher before CB compared to baseline, and is significantly reduced from before CB to all post-CB assessments. Reduction of negative affect and Attainment of certainty were the sole functions predicting the affective course during CB, and most prominently reported transdiagnostically. Assumptions of the etiological models were confirmed, suggesting that exposure and ritual prevention should be examined across disorders. As attainment of certainty seems to be predictive for the course of CB, this might be targeted in cognitive interventions.