Pré-publication, Document de travail: For most consumers, Corporate Social Responsibility is a credence attribute of products, which can be signaled either through a label certified by a third party, or via unsubstantiated claims used as part of a brand-building strategy. These claims may, in theory, be regulated by reputation mechanisms and the awareness of NGOs and activists. We use an experimental posted-offer market with sellers and buyers to compare the impact of these signalling strategies on market efficiency. Both third-party certification and the possibility of CSRrelated brand building give rise to a separating equilibrium. However, only third-party certification clearly produces efficiency gains, by increasing CSR investments. In markets where reputation matters little, unsubstantiated claims can generate a 'halo' effect on consumers, whereby the latter are nudged into paying more for the same level of CSR investments by firms.