Contemporary health policy in England places increasing emphasis on patient and public involvement (PPI) in health and health research. With regard to the latter, it has been suggested that PPI brings 'different' perspectives to research decision-making spaces, based on what has been referred to as 'experiential expertise'. This article presents findings from a qualitative study of PPI in cancer research settings in England. We argue that participants highlighted specific forms of expertise in their accounts about involvement, above and beyond experiential expertise, which they felt legitimated their claims to be credible participants within cancer research settings. We report here on the various strategies by which participants sought to accomplish this and highlight, in particular, a concomitant process of 'professionalization' of some within our group of participants. We discuss the significance of these findings in the context of recent debates around the status of experiential expertise.