This paper addresses treaty interpretation in the context of international organizations, with particular attention to interpreting
the founding or constitutive treaties of organizations. That not all interpretive rules are the same for all treaties is a
well-tried proposition (see e.g. Arnold McNair in BYIL 1930 and Joseph Weiler in EJIL 2010). The interest in different canons
of treaty interpretation generally hinges on the distinction of interpretative practices for different areas of substantive
law, such as investment treaties, or human rights treaties. What has been arguably under theorized is that interpretation
of the rules of organizations begs an additional question which does not arise with other treaties, that is: whether interpretation
of a treaty is even at issue. In other words, are UN Security Council resolutions a form of treaty law or do they amount to
‘international legislation’? Was the 1999 new ‘strategic concept’ of NATO a (highly ‘evolutive’) interpretation of the 1949
North Atlantic Treaty or a redefinition of powers and competences by an Organization making use of its compétence de la compétence?
In practice it seems these questions are mostly answered by upholding both the treaty character and the institutional law
character of such rules.
If and when the law of treaties is applied, interpretive practice vis-à-vis constitutive
treaties differs notably from general treaty interpretation on two counts. First, special weight is given to the ‘object and
purpose’ of the treaty by way of a general teleological approach to interpretation, rather than as a means to establish the
text’s ‘ordinary meaning’ per VCLT Article 31. Secondly, special weight is also given to the subsequent practice of the organization.
In contrast, classic interpretive methods that safeguard the State party’s ‘sovereign will’ - such as recourse to the travaux
préparatoires, party intention, and the subsequent practice of treaty parties - seem to have faded into the background. I
argue that these points of distinction are related to the extra layer of the organization’s legal order over the constituent
treaty. Or, put in doctrinal terms: these distinctions reflect a shift from a contractual to an institutional perspective
(or even ‘paradigm’) on constitutive treaties. As a result, it is the organization, not the States parties, that ends up taking
on any interpretative questions, and application of the classic law of treaties is limited.