This paper aims to contribute to the literature on social norms in IR. The intention is to further specify how social norms influence state behaviour. The empirical material consists of a case study of how the Czech Republic has participated in EU policy making in the field of environmental policy. The paper first discusses the different reasons for why member states adapt to EU norms: i.e. rational conditionality and constructivist social learning. The former of the two is viewed as typical for the pre-accession period whereas the latter is important for our understanding of the post-accession period. In order to outline the actual socialization process a detailed case study of how the Czech Republic has dealt with two different EU policy initiatives (REACH and the climate-energy package) is presented. The conclusions suggest that even if we can find indications of socialization the process is complicated by a strong presence of competing norms.