This article explores the genesis of the Common Market of the South (Mercosur), the regional integration scheme grouping Argentina, Brazil, Paraguay and Uruguay since 1991, and which Venezuela joined in 2006. The aim is to portray an accurate account of the diplomatic history of the foundation of Mercosur during the years 1989–1991. Methodologically, a case is made in favour of the use of oral history in the study of high politics. Argentina and Brazil's reading of the new international circumstances of the early 1990s are explored, and so are their respective international insertion strategies. The diplomatic negotiations leading to the 1991 Treaty of Asunción constitutive of Mercosur are dissected. The relationship between Mercosur and the so-called ACE-14 agreement concluded by Argentina and Brazil in the framework of the Latin American Integration Association is also illustrated, as are the negotiations to incorporate new members to the incipient common market. The final section suggests that, while there was an overall continuity in the integration project in the Southern Cone between 1985 and 1991, nonetheless Mercosur was a departure from previous objectives, timing and methodology. For good or ill, Mercosur was the creation of the neo-liberal governments of Presidents Menem of Argentina and Collor of Brazil and the result of broader changes at the international level.