The burden of diarrhoeal disease remains high in the developing world. Community-based safe drinking water programmes are being promoted as cost-effective interventions that will help reduce this illness burden. However, the effectiveness of these programmes remains under-investigated. The primary argument of this paper is that the biological exposure reductions underlying safe water interventions vary tremendously over space and time, and studies that only report results of intent-to-treat analyses cannot reveal why such programmes succeed or fail. The paper develops a stepwise evaluation framework to characterize, and so analyse, the technical, financial, social and behavioural factors that underlie exposure and mediate the impact of safe water investments. Relevant factors include physical performance of the water system, community capacity to maintain and manage the systems, and the time and budget constraints of households participating in the programme. The approach draws on the public health, community-based resource management, and household choice literatures to identify modifiable points of failure along the causal pathway to programme impact. The evaluation framework is used to assess the performance and impact of UVWaterworks, a community-based water purification system in rural Mexico, 5 years after the programme began. No impact on diarrhoea incidence was found in this case. The assessment method revealed that (a) household priorities and preferences were a key factor in maintaining exposure to safe drinking water sources, and therefore (b) user convenience was a primary leverage point for programme improvement. The findings indicate that a comprehensive examination of the many factors that influence the performance and impact of safe water programmes is necessary to elucidate why these programmes fail or succeed.