Purpose The ICECAP-A and EQ-5D-5L are two index measures appropriate for use in health research. Assessment of content validity allows understanding of whether a measure captures the most relevant and important aspects of a concept. This paper reports a qualitative assessment of the content validity and appropriateness for use of the eq-5D-5L and ICECAP-A measures, using novel methodology. Methods In-depth semi-structured interviews were conducted with research professionals in the UK and Australia. Informants were purposively sampled based on their professional role. Data were analysed in an iterative, thematic and constant comparative manner. A two stage investigation - the comparative direct approach - was developed to address the methodological challenges of the content validity research and allow rigorous assessment. Results Informants viewed the ICECAP-A as an assessment of the broader determinants of quality of life, but lacking in assessment of health-related determinants. The eq-5D-5L was viewed as offering good coverage of health determinants, but as lacking in assessment of these broader determinants. Informants held some concerns about the content or wording of the Self-care, Pain/Discomfort and Anxiety/Depression items (EQ-5D-5L) and the Enjoyment, Achievement and attachment items (ICECAP-A). Conclusion Using rigorous qualitative methodology the results suggest that the ICECAP-A and EQ-5D-5L hold acceptable levels of content validity and are appropriate for use in health research. This work adds expert opinion to the emerging body of research using patients and public to validate these measures.