This research article analyzes the responsibility of international organizations (IO) for acts performed ultra vires. For that purpose the methodology used was a review of academic documents on international Institutional Law in general and international responsibility in particular. Related Advisory Opinions by the International Court of Justice (ICJ) were studied as well. The document addresses some concepts and elements of the law of IOs, such as their powers and capabilities (interpretation and the capacity of creating other organs). The study applies broadly to any IO unless something different is prescribed in their respective constitutive treaties, but it refers mostly to the UNO and the UN system, because it is the most universal organization as well as the orga nization with the richest internal legislation. The conclusion reached is that there are no specific norms related to the legality of ultra vires acts, nor procedures for determining the validity of acts carried out by international organizations.