On the 1st of April 1999 Somerset Health Authority and Somerset County Council established a Joint Commissioning Board (JCB) to commission mental health services. Simultaneously, the Somerset Partnerships Health and Social Care NHS Trust was set-up as a combined health and social care services provider; for the first time in England, the majority of social services mental health staff transferred their employment to this Trust. The JCB commissioned an evaluation of the impact of these innovations on users and carers, staff and senior officers and members in the organisations concerned. In the course of the research, one concept frequently recurred as a source of both optimism and concern: 'culture'. However, the meanings attributed to this term varied significantly between stakeholders. After summarising the methodology and the results to date of the evaluation, this paper briefly reviews the conceptions of culture in the literature on organisations, and introduces a taxonomy for discussing culture. That taxonomy is then deployed to explore the ways in which the notion of 'culture' is being applied to and within the combined Trust in Somerset, and the implications of those applications for local practice and national policy around partnership.