BACKGROUND: An intervention trial of the 'SuperAmma' village-level intervention to promote handwashing with soap (HWWS) in rural India demonstrated substantial increases in HWWS amongst the target population. We carried out a process evaluation to assess the implementation of the intervention and the evidence that it had changed the perceived benefits and social norms associated with HWWS. The evaluation also aimed to inform the design of a streamlined shorter intervention and estimate scale up costs. METHODS: Intervention implementation was observed in 7 villages. Semi-structured interviews were conducted with the implementation team, village leaders and representatives of the target population. A questionnaire survey was administered in 174 households in intervention villages and 171 households in control villages to assess exposure to intervention activities, recall of intervention components and evidence that the intervention had produced changes in perceptions that were consistent with the intervention core messages. Costs were estimated for the intervention as delivered, as well as for a hypothetical scale-up to 1,000 villages. RESULTS: We found that the intervention was largely acceptable to the target population, maintained high fidelity (after some starting problems), and resulted in a high level of exposure to most components. There was a high recall of most intervention activities. Subjects in the intervention villages were more likely than those in control villages to cite reasons for HWWS that were in line with intervention messaging and to believe that HWWS was a social norm. There were no major differences between socio-economic and caste groups in exposure to intervention activities. Reducing the intervention from 4 to 2 contact days, in a scale up scenario, cut the estimated implementation cost from $2,293 to $1,097 per village. CONCLUSIONS: The SuperAmma intervention is capable of achieving good reach across men and women of varied social and economic status, is affordable, and has the potential to be effective at scale, provided that sufficient attention is given to ensuring the quality of intervention delivery.