This paper discusses the role of the so-called 'rules of the organization' in the draft Articles on the Responsibility of International Organizations (ARIO), as adopted by the International Law Commission (ILC) on second reading in 2011. Although the rules of the organization occupy a central place in the ARIO, the ILC has decided not to take a “clear-cut view” on their legal nature as either international law or internal law of the organization. This paper argues that the ILC's indecision has left the ARIO with a fluctuating scope of application concerning various provisions such as the attribution of conduct, the breach of an international obligation, the obligation to make reparation, and countermeasures against an international organization. The term of art 'rules of the organization' was developed by the ILC in its work on the law of treaties but has rarely been addressed in legal scholarship. Part 1 therefore first examines the legal nature of the different components of the so-called 'rules of the organization': the constituent instruments, the acts, and the established practice of the organization. While the constituent instruments are contracts between States at the moment of the creation of an international organization, it will be contended that they also operate as constitutions during the life of the organization, giving it the autonomy to create internal law in force between the subjects of its legal order, including its member States. In analysing the ARIO on second reading, Part 2 accordingly suggests reconceiving the rules of the organization as 'internal law' of the organization as long as it functions effectively, so as to appropriately reflect its constitutional autonomy for purposes of international responsibility.