European security is not the preserve of the European Union (EU). Other international organisations (IO) participate actively in the diffusion of European norms, ideas and practices. And yet, the impact of the EU-IOs interactions on domestic processes often fails to attract the attention of researchers. This article claims that the meaning ascribed to international norms in security matters is also constituted by the inter-organisational context in which the diffusion takes place. On the basis of the analysis of the EU's conditionality dialogue with the chosen Western Balkan states, the article examines, first, how the EU, in international justice, amplifies the prescriptive power of the International Criminal Tribunal for the former Yugoslavia (ICTY) and how conditionality induces full compliance in this area; second, how the EU, in security and defence, conveys NATO norms and standards, promotes technical and political cooperation with the Alliance and, thus, indirectly fosters NATO accession; and third, how the EU, in multilateral diplomacy, distorts the principles and practices of the Organisation for Security and Cooperation in Europe (OSCE) by expecting Western Balkan states to relinquish their commitment to sovereign equality, thereby stirring up confusion in diffusion patterns. The findings suggest that the EU-IOs interactions and norm trajectories contribute to explain diffusion patterns in international security matters.