Abstract Subsequent practice of international organizations plays an important role in the interpretation of their constituent instruments. Based on Articles 5 and 31(3) of the Vienna Convention on the Law of Treaties (VCLT), when such practices have become established or have been consented to by all States parties, they can be considered authoritative interpretations. The legal basis for considering practices of international organizations in other circumstances, however, is not entirely clear. The International Law Commission proposed that these practices may still be considered in the application of Articles 31 and 32 of the VCLT but left many issues undiscussed. This article argues that it is possible to consider international organizations’ practice in the application of Article 31 if a strong inference about the original intention of States parties can be drawn therefrom. To address difficulties involved in evaluating the probative value of international organizations’ practice, this article borrows a tool from probability theory—likelihood ratio—and shows that, absent a major change of circumstances, international organizations’ earlier practice can be seen as credible evidence of States parties’ original intention and considered in the application of Article 31.
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