Objective This study explored the role of psychosocial factors in predicting both membership to either an irritable bowel syndrome (IBS) or control group, and the severity of IBS symptoms. Methods A total of 149 participants (82 IBS and 67 controls) completed a battery of self-report inventories assessing disposition and environment, cognitive processes, and psychological distress. Logistic regression analyses were used to assess predictive contribution of psychosocial variables to IBS diagnosis. Hierarchical linear regression was then used to assess the relationship of psychosocial variables with the severity of IBS symptoms. Results Significant predictors of IBS were found to be alexithymia, the defectiveness/shame schema, and four coping dimensions (active coping, instrumental support, self-blame, and positive reframing), χ 2 (6, N =130)=46.99, p <.001, with predictive accuracy of 72%. Significant predictors of IBS symptom severity were two of the alexithymia subscales (difficulty identifying feelings, and difficulty describing feelings), gender, the schemas of defectiveness/shame and entitlement, and global psychological distress. This model predicted IBS severity F (6, 64)=16.94, p <.001 and accounted for 61.3% of the variability in IBS severity scores. Conclusion Psychosocial variables account for a large percentage of the differences between an IBS and control group, as well as the variance of symptoms experienced within the IBS group. Alexithymia and the defectiveness schema appear to be related to both IBS and symptom severity.