This article suggests that the arbitral use of jus cogens and erga omnes is slowly producing a ‘presumption of applicability’ in investor–state arbitration tribunals tasked with determining whether international law applies to investors. Looking backward from two recent counterclaims at tribunals established under investor–state dispute settlement (ISDS) clauses in international investment agreements that support our claim, the article first analyses the evolution and use of jus cogens in attaching rights-claims to non-State subjects of international law. Secondly, it explores the origins and uses of obligations erga omnes in attaching obligations-claims to non-State subjects of international law. It attempts to define the contours of jus cogens and erga omnes as they are deployed in recent ISDS case law and highlights the limits of each concept. The article concludes that there is a potential for a presumption of applicability in international investment arbitration for investor obligations and that this presumption can be sourced from general international law.