Background: Research about the psychometric properties of the Behavioural Appraisal Scales (BAS) in people with profound intellectual and multiple disabilities (PIMD) is limited. This study evaluates invariance in factor structure, item bias and convergent validity of the BAS. Methods: Data on the BAS from two studies (n = 25; n = 52) were analysed using the oblique multiple group method. The scale structure and item ordering were compared in the two groups. Convergent validity was assessed by correlating scores on the BAS with scores on two other instruments. Results: Of all items, 16-18% correlated stronger with other subscales of the BAS than the subscale they were originally assigned to. Scale structure and order of difficulty differed between groups. Correlations between the BAS and two other instruments varied from low to excellent (r = 0.48-0.85). Conclusions: The results support the construct validity of the BAS. Removing, reassigning and adapting items may enhance construct validity.