Interplays between international and domestic legal spheres have attracted increased attention in investor-State dispute settlement. From the treaty ratification process to award execution, constitutional norms play recurring roles before, during and after investment arbitrations. This contribution deals with the manner in which parties to such disputes can rely upon constitutional law or, more broadly speaking, domestic law. Notably, major hurdles to the application of domestic law in transnational fora have not necessarily constrained the arbitral profile of constitutional principles. This is because they may gain prominence through informal paths. Rather than directly applying constitutional law per se, tribunals may utilize other paths such as deferring to domestic interpretations of constitutional principles, or to constitutional procedures that appear, for example, to protect fair and equitable treatment. Reexamining recent case law through this lens of informal application, we can then envision other synergies that intermingle these regimes.