The legitimacy of certification for agricultural products depends on the belief that product labelling can provide information and guarantee the quality that consumers want. The neoclassic paradigm actually suggests that the problem of quality is to do with simple asymmetric information between economic agents. In our paper, however, we consider that the notion of quality is by no means objective: practices required (to obtain the given quality) and the credibility and legitimacy of quality control used in the different guarantee systems (to ensure standard compliance), constitute an institutionalised compromising device. This situation results from the balance of power and beliefs that exists between the organisations concerned. In this paper, we compare two different organisational mechanisms when examining the agricultural product standards designed to improve sustainable development: (i) the third party certification (TPC) is a mechanism that most public bodies recognise as being legitimate for the certification of sustainability standards; and (ii) the alternative mechanism of participatory guarantee systems (PGS), which is struggling to gain recognition from public authorities. Finally, we argue that the effectiveness of proximity and social control for guaranteeing sustainability standards in PGS seems just as credible and legitimate as the effectiveness of the independence and neutrality claimed by the TPC in the framework of international standards. In fact, TPC and PGS are alternative and complementary systems, rather than competitive systems, for implementing different sustainability standards.