Patient and public involvement (PPI) in health-care commissioning decisions has always been a contentious issue. However, the current moves towards Sustainability and Transformation Partnerships (STPs) in England's NHS are viewed as posing the risk of reducing the impact of current structures for PPI.To understand how different members in clinical commissioning groups (CCGs) understand PPI as currently functioning in their decision-making practices, and the implications of the STPs for it.Thematic analysis of 18 semi-structured interviews with CCG governing body voting members (e.g. clinicians and lay members), non-voting governing body members (e.g. Healthwatch representatives) and CCG staff with roles focussed on PPI, recruited from CCGs in South London STPs.There are contestations amongst CCG members regarding not only what PPI is, but also the role that it currently plays and could play in commissioning decision making in the context of STPs. Three main themes were identified: PPI is 'going out' into the community; PPI as a disruptive power; and PPI as co-production, a 'utopian dream'?Long-standing issues distinctive to PPI in NHS prioritization decisions are resurfacing with the moves towards STPs, particularly in relation to contradictions between the rhetoric of 'partnership' and reorganizations that foster more top-down control. The interviews reveal pervasive distrusts across a number of levels that are counterproductive to the collaborations upon which STPs rely. And it is argued that such distrust and contestations will continue until a formalized space for PPI in STP priority-setting is created.