Summary: This study analyzes the psychometric properties of the Spanish translation of the Matson Evaluation of Social Skills with Youngsters (MESSY; Matson, Rotatori & Helsel, 1983 ), which assesses the degree of appropriate social behavior. This social behavior self-report was applied to a sample of 634 adolescents between the ages of 12 and 17. The exploratory factor analysis isolated four factors: Aggressiveness/Antisocial Behavior, Social Skills/Assertiveness, Conceit/Haughtiness, and Loneliness/Social Anxiety, which accounted for the 33.28% variance. The internal consistency was high (α = .88). Correlations with similar self-reports, the Assertiveness Scale for Adolescents, the Teenage Inventory of Social Skills, and the Assertiveness Scale for Children and Adolescents, were statistically significant. Inappropriate social behavior measured with the MESSY correlated positively with the Psychoticism and Neuroticism scales, and negatively with the Extraversion scale of the Eysenck Personality Questionnaire. Female adolescents obtained lower scores in Aggressiveness/Antisocial Behavior and in Conceit/Haughtiness, and higher scores in Social Skills/Assertiveness, displaying greater appropriate social behavior than male adolescents. No significant differences were found for gender in Loneliness/Social Anxiety, the only factor in which a worsening with age and a significant gender× age interaction effect were found.