This article studies the mechanisms of socialization and re-socialization among national civil servants embedded in EU institutions. Applying a cognitive organizational theory approach, it is argued that national civil servants attending EU committees supplement pre-existing role perceptions with supranational roles under particular conditions. EU committees are seen as transformative institutions that accompany a partial re-socialization of the committee participants. The empirical data demonstrate that domestic civil servants become re-socialized due to their intensity of participation on EU committees. Based on survey and interview data on Danish and Swedish government officials who attend Council working parties (CWPs), the analysis reveals that the intensity of attendance on CWPs accompanies the enactment of supranational roles among the participants. Contrary to neo-functionalist assumptions, however, the length of participation on CWPs does not contribute to re-socialize the committee participants. The empirical analysis also demonstrates that supranational roles are indeed secondary to pre-existing national and sectoral roles. Hence, contrary to neo-functionalist arguments, the emergence of supranational roles does not replace pre-established roles.