The presence of overvalued ideas in obsessive–compulsive disorder (OCD) has been theoretically linked to poorer treatment outcome [Kozak, M. J. & Foa, E. B. (1994). Obsessions, overvalued ideas and delusions in obsessive–compulsive disorder. Behaviour Research and Therapy, 32, 343–353]. To date, no measures have been developed which quantitatively assess levels of overvalued ideas in obsessive–compulsives. The present studies examined the psychometric properties of a scale developed to measure this form of psychopathology, the Overvalued Ideas Scale (OVIS). In study 1, 102 patients diagnosed with OCD were administered a battery of instruments including the OVIS at baseline and two weeks later, prior to initiating treatment. Results indicate that the OVIS has adequate internal consistency reliability (coefficient α=0.88 at baseline), test–retest reliability (r=0.86) and interrater reliability (r=0.88). Moderate to high levels of convergent validity was found with measures of obsessive–compulsive symptoms, a single item assessment of overvalued ideas and psychotic symptoms. Medium levels of discriminant validity with measures of anxiety and depression was obtained in this study. Individuals determined to have high OVI showed greater stability of this pathology than those with lower OVI, suggesting that overvalued ideas are stable for extreme scorers. In study 2 a total of 40 patients participated who were diagnosed with OCD. The same battery of instruments was administered as in study 1, as well as the Beck Depression Inventory and Beck Anxiety Inventories. Results were similar to that obtained in study 1, including a relative lack of discriminant validity with self-report measures of depression and anxiety. It is suggested that further research with the OVIS may show predictive value in treatment outcome studies of OCD.