AbstractAdministered to mental health outpatients (N= 75) who were receiving Psychotherapy the Rational Behavior Inventory as well as several other scales of psychological adjustment prior to their first therapy session. Their therapists also completed several of these scales on their clients. This procedure was repeated for a subsample upon termination of treatment. Correlations were computed among measures, andt‐tests were conducted between pre‐ and posttreatment scores. Significant correlations were found between all measures in the expected directions, and significant pre‐ to postscale differences were identified consistent with the expected effects of treatment. These results are interpreted as initial evidence for the Rational Behavior Inventory's reliability and validity with a clinical group. Future research is suggested to examine the Rational Behavior Inventory's relationship to other measures of cognitive dysfun