This paper examines the legal nature of the ‘rules of international organizations’ as defined by the International Law Commission in its works on the law of treaties and on international responsibility. Part 1 introduces the debate with an example concerning the nature of un Security Council anti-terrorism resolutions. Part 2 challenges the four theories of the rules envisaged by scholarship. Part 3 is an attempt to examine the characteristics of the legal system produced by international organizations taking advantage of analytical jurisprudence, developing a theory of their legal nature defined as ‘dual legality’. Part 4 concludes by appraising the effects of the dual legality looking at the law of treaties, international responsibility and invalidity for ultra vires acts.