Vienna Convention on the Law of Treaties 1969

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The Vienna Convention on the Law of Treaties (VCLT) is the leading international Convention on treaties, which codifies the customary laws as to how states approach agreements, thus commonly being called ‘the treaty on treaties’. At current, the VCLT has been signed by 114 countries, although more states than this recognize and accede to parts of the Convention.

Why Introduced

A need for a single formal document on treaties was recognised in 1949, and the International Law Commission spent nearly twenty years creating and re-drafting the VCLT, which contains 85 Articles and was then submitted to the Vienna Conference for review in 1968 and 1969. As part of this process, the International Law Commission appointed four ‘Special Rapporteurs’, namely James Brierly, Hersch Lauterpacht, Gerald Fitzmaurice and Humphrey Waldock, who were prominent scholars on international law and who dedicated approximately fifteen years to the drafting and formulation of the VCLT drafts. The VCLT was subsequently adopted on the 22nd of May, 1969, although it did not become effective until the 27th of January, 1980.


Whilst many states already recognized customary laws governing how states could form agreements, the VCLT was intended to formalize and codify these customary laws, laying a foundation for how treaties are made and effected in international public law.

Thus, the VCLT includes provisions regarding how international treaties can be adopted, have their text authenticated, the definition and significance of a state consenting to be bound by the treaty, and how this may differ depending on whether the consent is given by way of signature, an exchange of instruments, ratification, acceptance, accession or approval, inter alia. Furthermore, the VCLT provides an approach for managing how a treaty may be approved with reservations (where the state agrees to a treaty but with certain exceptions), how treaties can enter into force and be provisionally applied, that such treaties ought be generally observed and applied by any countries which are a party to it in good faith (as per pacta sunt servanda). Further provisions provide for the interpretation of treaties, and how parties may seek to amend, modify, terminate or suspend treaties, alongside the possible procedural approaches for implementing ramifications for a country’s failure to comply.

Moreover, in accordance with the general principles of the rule of law, the VCLT provides that treaties ought not be retroactive, have territorial scope.


The VCLT is generally not considered to have made many substantive changes to the preceding customary laws on treaty making; rather its greatest change is in combining these various customs into a single binding legal document. Perhaps the only exception to this lies in the VCLT’s provisions on the approach and effect to agreeing to a treaty with reservations, as this is the area of customary law which was least harmonised prior to the VCLT’s implementation

Indeed, whilst the Convention disregards the validity of treaty provisions which run contrary to pre-emptory norms (fundamental ideas in international law which are recognised as universally binding and imperative; the ius cogens concept), it makes no comment on the appropriate substance for treaties, or indeed even provides examples of such pre-emptory norms.

Thus, whilst the VCLT is the most authoritative Convention in international public law regarding treaties, it is not exhaustive and possesses the flexibility to accommodate evolving approaches to treaty making without requiring formal revision or amendment. Furthermore, the benefits of codifying customary treaty law include the reduction of scope for dispute and doubt over treaties, in turn facilitating greater international co-operation and co-ordination, as well as serving as a strong foundation for the development and expansion of international public law.

Key Sections

Article 2(1) provides a definition for treaties:

‘…an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation’. Notably, per this definition, oral agreements between states are excluded from the scope of the VCLT.

Article 3 limits the scope of the VCLT to inter-state agreements, thus excluding agreements made between states and international organisations.

Article 6 considers the ability of states to form treaties, affirming that:

‘Every State possesses capacity to conclude treaties.’

Article 53 recognises the need to respect pre-emptory norms (ius cogens):

‘A treaty is void if, at the time of its conclusion, it conflicts with a peremptory norm of general international law. For the purposes of the present Convention, a peremptory norm of general international law is a norm accepted and recognized by the international community of states as a whole as a norm from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character.’

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