This article examines the European Union's Committee of Permanent Representatives, or COREPER, a group composed of the EU permanent representatives (permreps) and responsible for preparing upcoming ministerial meetings of the Council. As the heart of everyday decision making in the EU, COREPER is a key laboratory to test whether and how national officials become socialized into a Brussels-based collective Culture and what difference this makes for EU negotiations. The key scope conditions for COREPER socialization are high issue density/ intensity and insulation from domestic politics. COREPER also displays a range of socialization mechanisms, including strategic calculation, role playing, and normative suasion. Based on extensive interview data and a detailed case study of negotiations for a controversial EU citizenship directive, this article documents a socialization pathway in COREPER marked by adherence to a set of norm-guided rules and principled beliefs in collectively legitimating arguments and making decisions. COREPER socialization does not indicate a pattern of national identities being replaced or subsumed; rather, the evidence points to a socialization process based on a "logic of appropriateness" and an expanded conception of the self.