Background: The Personal Outcomes Scale (POS) is a scale developed to measure quality of life of adults (18+) with intellectual disability. Previous studies have reported good fit for Spanish and Portuguese language versions of POS. Aims: This study aimed to evaluate the factor structure of the English language version of POS when used to measure the quality of life of adults (18+) with intellectual disability in the UK. Materials and Methods: Analysis was conducted on POS data from 310 adults with an intellectual disability. First and second order factor models and multi-level models were used to assess fit. Results: There was poor fit to the data for all tested models. We estimated that 23% of variance in POS scores was accounted for by interviewer cluster. Discussion: This was the first UK-based evaluation of POS and our data did not confirm the factor structure of the POS measure. The identification of systematic variability within the dataset indicates that inter-rater reliability is a potential limitation of the POS tool. Conclusion: Further research is needed to investigate inter-rater reliability of POS interviewers and to explore factor structure.