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People with severe mental illness (SMI) have high rates of substance use disorders. The Transtheoretical Model (TTM) is a framework for understanding behavior change. There are five leading measures associated with the TTM -University of Rhode Island Change Assessment, Processes of Change Scale, Decisional Balance Scale, Abstinence Self-Efficacy Scale, and Temptation to Use Drugs Scale. While these measures have been found to be reliable and valid in primary substance abusers, it is unknown if these measures are relevant in people with co-occurring SMI and substance use disorders (SUD). We evaluated the psychometric properties of these measures in a sample (n=240) of people with co-occurring disorders. Participants met DSM-IV criteria for schizophrenia/schizoaffective disorder or non-psychotic affective disorder, and current cocaine dependence or cocaine dependence in remission. All subscales showed good reliability and validity in the total sample. Analyses within diagnostic groups showed good reliability and validity in most groups, with some falling off in the affective disorders and remitted cocaine dependence groups. Overall, findings support the use of these measures in people with co-occurring disorders. Keywordsserious mental illness; substance abuse; transtheoretical model; psychometrics Corresponding Author: Melissa Nidecker, Ph.D., Postdoctoral Fellow, VA Maryland Healthcare System, BT/MIRECC, 10 North Green Street, Baltimore, MD 21201, Phone: 410-605-7000 ext. 4734, Melissa.nidecker@va.gov. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public Access NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript IntroductionSubstance abuse is a critical and devastating health problem in individuals with severe mental illness (SMI). Nearly 50% of all persons with SMI have met criteria for a lifetime substance use disorder (SUD; Barry, Fleming, & Grennley, 1995;Mueser, Yarnold, & Bellack, 1992). SUDs are especially harmful for people with SMI, who are already severely disadvantaged by impairments that are part of their mental illness. People with co-occurring SMI and SUDs show more medication noncompliance, symptom exacerbations, and re-hospitalizations than do people with SMI only, as well as poor social adjustment and an overall poorer prognosis (Drake, Mueser, Clark, & Wallach, 1996;Cournos, et al., 1991). Although treatment for people with co-occurring SMI and SUDs is complex, interventions for substance abuse that emphasize long-term, integrated treatment that is tailored to the special needs of people with SMI have been shown to b...