Abstract Objective To investigate (i) how the SLIMMER intervention was delivered and received in Dutch primary health care and (ii) how this could explain intervention effectiveness. Design A randomised controlled trial was conducted and subjects were randomly allocated to the intervention (10-month combined dietary and physical activity intervention) or the control group. A process evaluation including quantitative and qualitative methods was conducted. Data on process indicators (recruitment, reach, dose received, acceptability, implementation integrity and applicability) were collected via semi-structured interviews with health-care professionals ( n 45) and intervention participant questionnaires ( n 155). Setting SLIMMER was implemented in Dutch primary health care in twenty-five general practices, eleven dietitians, nine physiotherapist practices and fifteen sports clubs. Subjects Subjects at increased risk of developing type 2 diabetes were included. Results It was possible to recruit the intended high-risk population (response rate 54 %) and the SLIMMER intervention was very well received by both participants and health-care professionals (mean acceptability rating of 82 and 80, respectively). The intervention programme was to a large extent implemented as planned and was applicable in Dutch primary health care. Higher dose received and participant acceptability were related to improved health outcomes and dietary behaviour, but not to physical activity behaviour. Conclusions The present study showed that it is feasible to implement a diabetes prevention intervention in Dutch primary health care. Higher dose received and participant acceptability were associated with improved health outcomes and dietary behaviour. Using an extensive process evaluation plan to gain insight into how an intervention is delivered and received is a valuable way of identifying intervention components that contribute to implementation integrity and effective prevention of type 2 diabetes in primary health care.