The role of policy evaluation in social experimentation, between instrumentation and instrumentalisation. RSA case study : experimentation and evaluation. This article tackles the issue of policy evaluation in a context of social experimentations through the case studies of the RSA experimentations. It analyzes how the launching of the experimentation and its evaluation, its implementation and its usages are influenced by the liaison of interests, institutional dynamics and beliefs systems. The procedure turns out to be negotiated and invested by the main actors, and the evaluation pressured by political challenges, and therefore altering the knowledge that it produces. As a result, this procedure gives some hard evidence to the decision-makers of the social acceptance of the scheme, of its feasibility, operationality and efficiency. These results represent a crucial political argument that legitimizes the policy making, the experimental scheme and the actors that implement it. Therefore it questions how the instrumentation (or even the instrumentalization) of the experimentation and its evaluation, contributes to modifying the way in which public action is elaborated, legitimated and led.