This article addresses a subject that is relevant to the jurisdiction of a tribunal in investment cases, namely the nationality of the natural person. More specifically, this article will describe what the standard of proof is for the claimant who wants to prove this nationality. This subject has not been addressed extensively in literature and, in a globalised business environment, together with the fact that double or even triple nationalities have become more common, it is a relevant topic. This article will describe what the current standard of proof is that rests on the claimant, but also different standards that have been suggested in legal writing.