Having analysed the factual background and procedural development of the ICSID case Pacifi c Rim v. El Salvador, this article suggests an argumentative approach that is alternative to the one adopted by the arbitrators in the final award and based on domestic constitutional law arguments. This approach, inspired by the practice of transnational human rights litigation, aims to provide a tool for introducing human rights into the investor-state adjudication system, and to favour the (functional) ‘constitutionalisation of international law’ (CIL). Under this approach, international arbitrators, when applying domestic law, would apply not only the relevant provisions of ordinary law, but the domestic constitutional system as a whole, taking into account its specific axiological hierarchy. The article finally explores the broader impact that this approach could have on CIL, engaging with this task through the lenses of the theory of societal constitutionalism.