The mechanisms that enable the joint participation of the European Community (EC) and its twenty seven sovereign member states in multilateral bargains are much more complex than what the three pillar architecture suggests. Nevertheless, the influence of inter-branch negotiations on the EC/EU’s international actorness4 still remains a gray area between law and political science. An under-explored aspect regards the distribution of competences – defined as authority to undertake negotiations with third states and international organizations – within the Community institutional framework. Such competences, or powers, may be exclusive to member states (MSs), exclusive to the EC represented by the Commission, or shared by both. The last situation, known as mixed participation (or mixity) is the key variable of this study. This thesis seeks to shed light on EU actorness under mixity conditions by investigating how and to what extent the distribution of competences between the European Commission and the Council of Ministers influences the performance of the EC in the negotiation of certain types of global environmental agreements. The methodology consists of comparative analysis of three empirical cases related to the exploitation of living marine resources (fisheries). A two-level approach grounded in rational choice institutionalism is adopted, dividing the study in two main parts: the first focuses on EU level bargains (L1) in order to explain the different kinds of contracts (mandate) established between the Council and the Commission through a principal-agent relationship. The independent variables taken into account are Council‟s preferences, Commission’s preferences, and the extent of the knowledge about ecological processes impacted by the forthcoming policy decisions stemming from the agreements. This last variable was called „environmental scientific uncertainty‟ (ESU), as this term is already used by environmental economists. The second part of the thesis addresses the negotiation of global agreements to which the EC was part, either exclusively or together with MSs. As there is a shift in the level of analysis the mandate – the dependent variable at L1 – becomes an independent variable at L2. The goal is now to explain: a) the effects of mixity as opposed to EC exclusive competence on the EU’s actorness. The underlying argument is that the implications of mixity go beyond European integration; in fact they are critical to the strengthening of EU agency vis-à-vis other players. They appear as a valuable institutional mechanism in domains marked by scarcity of reliable scientific data about ecological processes (as in ESU). The three cases are studied in both L1 and L2 and relate to fisheries policies: a) the Agreement to Promote Compliance by Fishing Vessels on the High Seas (1995), under the FAO framework; b) the Agreement on Straddling and Highly Migratory Fish Stocks (1995), Under the United Nations Convention on the Law of the Sea (UNCLOS) framework and c) The Jakarta Mandate, an agreement on Biodiversity of Marine and Coastal Areas (1995), under the Convention on Biological Diversity framework. Data stem mainly from Law databases and official journals. Results shall shed light on the external impact of institutional arrangements within the Community, and their relevance to the study of the EU’s international relations.