The Motivation Assessment Scale (MAS) and the Questions About Behavioral Function (QABF) are frequently used to assess the learned function of challenging behaviour in people with intellectual disability (ID). The aim was to explore and compare the psychometric properties of the MAS and the QABF.Seventy adults with ID and challenging behaviour and their disability support workers participated in the study. Support workers completed the MAS and QABF regarding a challenging behaviour that they identified as causing most concern.Both measures demonstrated good internal consistency. Based on the intra-class correlation coefficient, inter-rater reliability of the MAS and QABF was acceptable for sub-scale scores, but not for individual items. Convergent validity, as reflected by correlations between functionally analogous scales, was satisfactory, but there was low agreement between the MAS and QABF on the function of challenging behaviour. Factor analysis of the QABF revealed factors that clearly corresponded to the five factors reported by the developers, four of which were well determined. Similar analyses of the MAS yielded a four-factor solution, however, only one factor was well determined.The psychometric properties of the MAS and QABF were similar, and item-by-item reliability was problematic. The results suggest that both measures may prove unreliable for assessing the function of challenging behaviour among adults with ID. In developing interventions to address challenging behaviour, other techniques (e.g. observations) should be used to supplement information from these measures.