To improve access to maternal health services, Benin introduced in 2009 a user fee exemption policy for caesarean sections. Similar to other low- and middle-income countries, its implementation showed mixed results. Our study aimed at understanding why and in which circumstances the implementation of this policy in hospitals succeeded or failed. We adopted the realist evaluation approach and tested the initial programme theory through a multiple embedded case study design. We selected two hospitals with contrastive outcomes. We used data from 52 semi-structured interviews, a patient exit survey, a costing study of caesarean section and an analysis of financial flows. In the analysis, we used the intervention-context-actor-mechanism-outcome configuration heuristic. We identified two main causal pathways. First, in the state-owned hospital, which has a public-oriented but administrative management system, and where citizens demand accountability through various channels, the implementation process was effective. In the non-state-owned hospital, managers were guided by organizational financial interests more than by the inherent social value of the policy, there was a perceived lack of enforcement and the implementation was poor. We found that trust, perceived coercion, adherence to policy goals, perceived financial incentives and fairness in their allocation drive compliance, persuasion, positive responses to incentives and self-efficacy at the operational level to generate the policy implementation outcomes. Compliance with the policy depended on enforcement by hierarchical authority and bottom-up pressure. Persuasion depended on the alignment of the policy with personal and organizational values. Incentives may determine the adoption if they influence the local stakeholder's revenue are trustworthy and perceived as fairly allocated. Failure to anticipate the differential responses of implementers will prevent the proper implementation of user fee exemption policies and similar universal health coverage reforms.