Occidental nation-states’ democratic principles have been progressively influencing the way they conduct their international relations. Parliamentary actors, for instance, have been organizing transnational forums in order to debate their perspectives on foreign policy. In Latin America, nevertheless, this trend is limited by the presidential tradition and the intergovernmental character of regionalism. How, thus, can the creation of a parliament in Mercosur in 2006 be explained ? This paper aims to evaluate the reasons for the Mercosur Parliament’s construction beyond official discourse. The analysis, based on the theoretical framework of the « 3 Is » and qualitative interviews, identifies factors related to institutions, interests and ideas which have had a joint role in this political outcome.