Whether the characterization of international law as a legal system grounded in state consent has ever been true is open to discussion. The law of treaties, however, is commonly seen as ‘a bastion of consensualism’. It is true that this sense of confidence has never sat easily with treaty interpretation. Despite the lip service sometimes paid to the fiction of the common intention of the parties, the official doctrine of treaty interpretation rests on the primacy of the terms of the treaty. The joint interpretation of the treaty by its parties, however, was generally seen as having a conclusive effect. This state of affairs was recently shaken by the International Law Commission in its work on subsequent agreements and subsequent practice. The paper tries to find out what made this attempted change in the relatively well-established legal regime of authentic treaty interpretation possible and whether this attempt is likely to be successful.