I model the participation of firms in a voluntary agreement as a costly certification process whereby a firm informs the Regulator of its pollution intensity. Without this knowledge, the Regulator imposes the same tax on all firms in a heterogeneous industry, unduly hurting the clean ones with the lowest intensity. Certification allows clean firms to get a tax rebate. It also entails an informational externality as the dispersion of types decreases within the pool of non-participating firms, following an unraveling process. Because participation is a firm’s private decision, there is such a thing as a bad voluntary agreement.