In the practice of modern international law, if a certain understanding is advanced as the correct interpretation of a treaty provision, the proposition is assessed using the rules of interpretation laid down in the 1969 Vienna Convention on the Law of Treaties, Articles 31–33. This article is concerned with the relationship held in such an interpretation process between the preparatory work of a treaty – described as a supplementary means of interpretation in Article 32 – and the three primary means of interpretation that can be used by an interpreter citing Article 31. Judged by the wordings of Articles 31 and 32, the relationship between the primary and the supplementary means of interpretation is hierarchical. As a consequence, preparatory work may be used to determine the correct meaning of a treaty provision, only on the condition that in the earlier stages of the interpretation process, the application of Article 31 has been found to leave the meaning of the interpreted treaty ‘ambiguous or obscure’, or to lead to a result ‘which is manifestly absurd or unreasonable’. Several scholars in the area have recently argued for granting the preparatory work of a treaty a more prominent role in the interpretation process. As this article will show, such a claim builds on arguments that do not stand up to careful legal analysis. It should, therefore, be discarded as unsound.