Objectives: This paper presents the results of a realist evaluation that aimed to understand how, why and under what circumstances a Red Cross (RC) capacity-building intervention influences the motivation and the performance of RC community health volunteers involved in the delivery of an immunisation programme in Kampala, Uganda. Method: Given the complexity of the intervention, we adopted realist evaluation as our methodological approach and the case study as our study design. Data collection included document review, participant observation and interviews. The constant comparative method was used for the analysis. Two contrasted cases were selected within the five Kampala districts. Each case covers the management of the immunisation programme implemented at a RC branch. In each case, a programme manager and 15 RC volunteers were interviewed. The selection of the volunteers was purposive. Results: We found that a capacity-building programme including supervision supportive of autonomy, skills and knowledge enhancement, and adapted to the different subgroups of volunteers, leads to satisfaction of the three key drivers of volunteer motivation: feelings of autonomy, competence and connectedness. This contributes to higher retention, and better task performance and wellbeing among the volunteers. Enabling contextual conditions include the responsiveness of the Uganda Red Cross Society (URCS) to community needs, and recognition of the work of the volunteers, from the URCS and the community. Conclusions: A management approach that caters for the different motivational states and changing needs of the volunteers will lead to better performance. The findings will inform not only the management of community health volunteers, but also the management of all kinds of health workers.