This paper focuses on a public policy measure to fight obesity in order to highlight how trust in such devices is built. Achieving the required conditions for trust is all the more challenging when policy instruments are intended to trigger market dynamics-as in the case of France's voluntary charters for improving nutrition, studied here. Far from being guaranteed by the sole support of public authorities, the effectiveness of such a tool depends, in a decisive way, on its ability to align the interests of the parties it brings into play (public authorities as well as economic and industrial actors). However, this alignment of interests is undermined by the very management of the device, which, guided by a concern to preserve its reputation, jeopardizes its effectiveness by regularly contributing to the misalignment of the parties’ interests.