My proposal aims at analyzing the mechanisms of “impact assessment” as a tool of environmental governance, more specifically “sustainability impact assessment” (SIA) of trade agreements created by the European Union as a way of ensuring that trade outcomes comply with sustainable development principles. Sustainable development and its three dimensions – economic, environmental and social – have become a guiding principle of the international community, and there is a general assumption that trade can work for the promotion of this goal - poverty is seen as one of the main causes of environmental degradation, and an increase in international trade as one alternative to alleviate “pollution of poverty”. Nevertheless, while the economic benefits of an open trading system are clearer, the social and environmental outcomes of trade agreements are still overlooked. Thus, in order to bridge the gap between commitment and compliance, mechanisms that bring international environmental norms into deliberations must be a part of the institutional setting of environmental governance. The EU SIA emerged after other models of impact assessment, but its model extended the scope of the analysis also to the partners and to the social and economic sphere apart from the environmental one. SIAs final recommendations are not binding, but enhance public dialogue and can provide better governance in two ways: firstly, by providing data and information on the effects of trade policy on all involved actors; second, by enhancing public debate and generating indirect effects on trade negotiations, based on the findings of the studies, even if the negotiating positions are not directly modified. This paper will take a pragmatic approach in this issue, analyzing a concrete SIA – prepared for the negotiation of the Association Agreement with MERCOSUR - and its criteria and final recommendations as a way of promoting environmental governance.