The validity of the Treatment Motivation Scales for outpatient offender treatment (TMS-F), a self-report questionnaire with scales for the motivation of patients to engage in the treatment and six cognitive and emotional determinants of this motivation, is evaluated in two studies. In Study 1 (N = 620), the "construct validity" of the TMS-F is investigated applying a multitrait-multimethod design with a therapist-rating instrument as the criterion method. All scales were found to have adequate convergent validity and acceptable discriminant validity. In Study 2 (N = 328), the "criterion validity" of the TMS-F is addressed. Applying covariance structure analysis, the instrument is found to predict therapist ratings of the treatment engagement of patients to a substantial degree. Treatment engagement is best predicted by the scales for Motivation to Engage in the Treatment and Perception of the Suitability of the Treatment and not by the scales for Distress and the Perceived Legal Pressure. (Contains 3 figures and 6 tables.)