Abstract***: This article reports the findings of a project entitled ‘The participation of members in mutual businesses’. A previous project developed a theoretical model of what makes people participate, focusing on the participation of public service users in council housing and social care services. The current project builds on this work, applying the ‘mutual incentives model’ to a population sample of area committee members and a random sample of non-participant members of a very large UK consumer co-operative, the Co-operative Group. Two arguments are presented as to why such research is needed. First, member participation in co-operative and mutual businesses is becoming an important issue both for this sector and more generally for public policy. Second, a comparison between a public services setting and a co-operative setting enables us to extend and further test the theoretical model. Two main features of the model are outlined: a ‘mutual incentives theory’ that goes beyond other models to combine individualistic and collectivistic motivations, and the ‘participation chain’, a synthesis of existing knowledge that joins motivations to three ‘links’ that we call ‘resources’, ‘mobilization’ and ‘dynamics’. The article then summarizes the project methodology, and reports the main findings. As in the public services project, on the ‘demand’ side, collectivistic incentives prove to be dominant over individualistic, but with some individual ‘internal’ benefits also being important. On the ‘supply’ side, skills derived from previous experience were important, as were a positive evaluation of opportunities to participate, and recruitment through existing networks. We then compare the findings to those from the public service users and from a regional co-operative society; Oxford, Swindon and Gloucester Co-op. Collective motivations are dominant in all three datasets, but are shown to vary in interesting ways that have important implications for member participation strategies.