As an alternative to materialist explanations of regionalism in South America, this essay offers a constructivist approach that assesses the constitutive role of ideas related to collective identity, collective knowledge and structure of roles as three ideational variables to understand the engagement of South American states with the Andean Community and Mercosur regionalist projects. According to these variables, which are taken from A. Wendt’s study of international culture, this essay addresses four sources of collective identity ?interdependence, common fate, homogeneity and self-restraint— and explains the structure of collective knowledge constituted by neo-liberalism and the strategy of ‘open regionalism’. It also assesses the structure of rules that defines the types of international culture of anarchy present in South American regionalist projects. The essay shows the ability of the constructivist approach to explain the development of these projects amidst low levels of regional cohesion, highlighting ideational dimensions often dismissed by materialist analysis