Background: There is little information on the psychometric properties of instruments for assessing family care burden in adults with intellectual disabilities (ID). The aim of this study is therefore to analyse the usefulness of the 'Subjective and Objective Family Burden Interview' (SOFBI) in the assessment of principal caregivers in Spain. Methods: The SOFBI was administered to 166 principal caregivers of adults with ID in a vocational centre. The psychometric analysis included: internal consistency, inter-rater and test-retest reliability, construct validity, convergent validity with the World Health Organization's Disability Assessment Schedule II, and feasibility. Results: The Cronbach's alpha was 0.88 for the overall interview and always above 0.7 in the quantitative subdomains. The Kappa coefficients for test-retest were between 0.5 and 0.8, whereas inter-rater agreement was nearly perfect. Maximum-likelihood factor analysis showed four well-defined factors, which fitted the previously designed domains. Feasibility was also good. Conclusions: The SOFBI is a multi-domain, modular instrument which is feasible, reliable and valid for measuring the burden of family caregivers to adults with ID living in the community.