The MOX Plant and IJzeren Rijn disputes illustrate the growing problem of concurrent jurisdiction between international courts and tribunals and the ECJ. This article argues that in cases in which Community law is involved in a dispute between two EC member states, international courts and tribunals must accept the exclusive jurisdiction of the ECJ under Article 292 of the EC Treaty to decide these cases. However, only the UNCLOS arbitral tribunal in the MOX Plant case stayed the proceedings and requested that the parties first find out whether the ECJ had jurisdiction, whereas the OSPAR as well as the IJzeren Rijn arbitral tribunals rendered their awards despite the implications of Article 292. Thus it appears that every arbitral tribunal decides the issue of Article 292 as it sees fit. This situation, it is argued, requires the creation of some sort of hierarchy between the growing number of international courts and tribunals in order to co-ordinate and harmonize their decisions so as to avoid a fragmentation of international law.