Affect instability, a core symptom of several major mental illnesses, contributes to high levels of subjective distress and impaired self-care abilities. The findings of previous studies have shown that, in asymptomatic samples, the organizational properties of the self-concept impact short-term affect regulation. In this study, that work is extended to examine the association between the organizational properties of the self-concept and instability of affect in adults with a major mental illness. Subjects were 19 adults with a major mental illness (clinical) and 10 asymptomatic adults. Zajonc's card-sorting task was used to measure two organizational properties: (a) differentiation–the number of attributes included in the self-concept, and (b) unity–the degree of interdependence among the attributes. An experience sampling procedure was used to obtain 50 measures of affect across a 10-day period. The clinical group experienced more instability of negative affects than the asymptomatic group. Furthermore, the clinical group had lower differentiation and higher unity of the self-concept than the asymptomatic controls, although both organizational properties were related to education. In the total sample, differentiation was not related to instability of affect. Unity accounted for 21% of the variance in instability of negative affect. ©1995 John Wiley & Sons, Inc.