The psychometric properties of a new 28-item self-report measure of mentalization, the Mentalization Scale (MentS), were examined in 2 studies: with a sample of employed adults and university students (N1 = 288 + 278) and with a sample of persons with borderline personality disorder (BPD) and matched controls (N2 = 62 + 62). Besides the MentS, both studies employed measures of attachment and the Big Five; Study 1 also included assessments of empathy and emotional intelligence. MentS whole-scale internal consistency was good in the community and acceptable in the clinical sample (α = .84 and .75, respectively). A principal components analysis of Study 1 data yielded 3 interpretable factors, or subscales: Self-Related Mentalization (MentS-S), Other-Related Mentalization (MentS-O), and Motivation to Mentalize (MentS-M). These showed acceptable reliabilities (α = .74-.79), except for MentS-M in the clinical sample (α = .60). MentS scores further exhibited a coherent pattern of correlations with cognate constructs and the Big Five, relating positively to empathy, trait and ability emotional intelligence, openness, extraversion, and conscientiousness, and negatively to attachment avoidance and anxiety, and neuroticism. Persons with BPD scored significantly lower on MentS total and MentS-S. The proposed scale is thus deemed suitable for quick, yet meaningful, assessments of mentalization in both individual differences research and clinical contexts.