This monograph seeks to examine the motivations for the European Union’s (EU) policy towards the Common Market of the South (Mercosur), the EU’s most important relationship with another regional economic integration organisation. This monograph argues that the dominant explanations in the literature -- balancing the US, global aspirations, being an external federator, long-standing economic and cultural ties, economic interdependence, and the Europeanization of Spanish and Portuguese national foreign policies – fail to adequately explain the EU’s policy. In particular, these accounts tend to infer the EU’s motives from its activity. Drawing extensive primary documents, this monograph argues that the major developments in the relationship -- the 1992 Inter-institutional Agreement and the 1995 Europe Mercosur Inter-regional Framework Cooperation Agreement – were initiated by Mercosur and supported mainly by Spain. This means that rather than the EU pursuing a strategy, as implied by most of the existing literature, the EU was largely responsive.