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Progress in clinical research and in empirically supported interventions in the area of psychopathology in intellectual disabilities (ID) depends on high-quality assessment instruments. To this end, psychometric properties of four instruments were examined: the "Aberrant Behavior Checklist" (ABC), the "Assessment of Dual Diagnosis" (ADD), the "Anxiety, Depression and Mood Scale" (ADAMS), and the "Social Performance Survey Schedule" (SPSS). Data were collected in two community-based groups of adults with mild to profound ID (n = 263). Subscale reliability (internal consistency) ranged from fair to excellent for the ABC, the ADAMS, and the SPSS (mean coefficient [alpha] across ABC subscales was 0.87 (ranging from fair to excellent), the ADAMS subscales 0.83 (ranging from fair to good), and the SPSS subscales 0.91 (range from good to excellent). The ADD subscales had generally lower reliability scores with a mean of 0.59 (ranging from unacceptable to good). Convergent and discriminant validity was determined by bivariate Spearman rho correlations between subscales of one instrument and the subscales of the other three instruments. For the most part, all four instruments showed solid convergent and discriminant validity. To examine the factorial validity, Confirmatory Factor Analyses (CFA) were attempted with the inter-item covariance matrix of each instrument. Generally, the data did not show good fits with the measurement models for the SPSS, ABC, or the ADAMS (CFA analyses with the ADD would not converge). However, most of the items on these three instruments had significant loadings on their respective factors. (Contains 4 tables.)