The issue of whether an international judicial forum may entertain the claim of a dual national against a state to which he owes allegiance, is an old one. What is new, however, is the challenge by the Iran-United States Claims Tribunal of the traditionally held view that, in the absence of a clear mandate and where customary international law is exclusively invoked, the forum may not do so. Such a claim is admissible, says the Tribunal, provided the claimant is able to demonstrate: (a) that at all relevant times his “dominant and effective” nationality was the one on which he now relies; and (b) that the rights in question, those for the asserted breach of which he seeks redress, were not secured because of his other, non-dominant, nationality. Leaving for another day the questionability of the challenge, the present article offers to examine some of the more important aspects of the second test, as expounded and applied by the Tribunal.