On 7 May 2026, Philippine President Ferdinand Marcos Jr., in his capacity as ASEAN Chair, convened a trilateral meeting at Shangri-La Mactan in Cebu with Thai Prime Minister Anutin Charnvirakul and Cambodian Prime Minister Hun Manet. On 8 May, Anutin posted on Facebook that the meeting and Cambodia’s acknowledgement that it took place “can be regarded as clear evidence that Cambodia has officially acknowledged Thailand’s decision” to terminate the 2001 Memorandum of Understanding on overlapping maritime claims.
The Vienna Convention on the Law of Treaties, which both states treat as customary international law, requires notification of treaty termination to be made in writing, to specify the measure proposed and the reasons, and to be carried out through an instrument signed by the Head of State, the Head of Government, or the Minister for Foreign Affairs. The text of Article 67, paragraph 1, runs to one sentence: the notification must be made in writing. Article 65 requires the writing to indicate “the measure proposed to be taken with respect to the treaty and the reasons therefor.”
MOU 2001 has no termination clause. Its operative text runs to five articles and a signature block; none provides for termination, withdrawal, denunciation, or expiry. Article 56 of the Vienna Convention provides that a treaty containing no termination provision is not subject to denunciation or withdrawal unless either the parties intended to admit that possibility or a right of denunciation is implied by the nature of the treaty. Where one of those conditions is established, the article requires twelve months’ notice of intention to withdraw, in addition to the Article 65 and 67 procedural requirements. The 5 May cabinet act addressed neither condition and issued no such notice.
Cambodia deposited its instrument of ratification of UNCLOS with the United Nations Secretary-General on 6 February 2026. The Convention entered into force for Cambodia on 8 March 2026. The depositary notification carried a full Article 298 exclusion declaration structurally identical to the declaration Thailand filed at its own ratification in 2011: both states excluded all three categories of disputes from Part XV Section 2 binding procedures, including maritime boundary delimitation. Both therefore activated, in identical terms, the proviso embedded in Article 298(1)(a)(i) itself: a state that files the exclusion must still accept compulsory conciliation under Annex V Section 2 when another party to the dispute requests it. Within hours of the Thai cabinet vote on 5 May 2026 to terminate MOU 2001, Cambodia’s Deputy Prime Minister and Foreign Minister Prak Sokhonn announced Cambodia’s invocation of the Annex V Section 2 mechanism. The instrument Cambodia would use had been deposited at the Treaty Office three months earlier.
On 5 May, Thai government spokesperson Rachada Dhnadirek announced the cabinet’s decision through the official thailand.go.th channel. The same day, Anutin told Thai media that Thailand “would formally notify Cambodia of the decision before it takes effect”. On 7 May, he told the Thai press that Thailand “will formally notify Cambodia through an official letter”. On 8 May, his Facebook post reframed the requirement. The trilateral at Cebu, and Cambodia’s acknowledgement that it had occurred, were the formal notification.
Sihasak Phuangketkeow, Thailand’s foreign minister, did not reach the same conclusion. Sihasak told Thai media that the UNCLOS process must begin with bilateral negotiations and that “various steps must be taken before such a process begins.” His statement aligned with neither the prime minister’s earlier promise of formal notification by letter nor the prime minister’s later claim that the meeting had already satisfied it. It also did not align with the text of Article 298(1)(a)(i), which contains no precondition that bilateral negotiations precede a request for conciliation. The proviso requires that “no agreement within a reasonable period of time” has been reached between the parties. It does not give the responding state a right to define what “reasonable” means by requesting another round.
On 6 May, Cambodia transmitted a diplomatic note to the Thai side formally notifying its intention to invoke compulsory conciliation under UNCLOS. The note serves as the written notification of institution of proceedings that Annex V Section 2 requires. A senior Cambodian official, in remarks carried by Khmer Times, the Phnom Penh Post, and the Cambodian government’s official news channel in Khmer, stated in mid-May that Cambodia “has not yet received any formal notification from the Thai side regarding the termination of the 2001 Memorandum of Understanding, in accordance with Articles 65 and 67 of the Vienna Convention on the Law of Treaties.”
On 12 May, Hun Sen, in his capacity as Acting Head of State while King Norodom Sihamoni was abroad, directed the Royal Government not to engage in bilateral negotiations with Thailand on maritime issues and to proceed directly to UNCLOS mechanisms without waiting for any agreement from the Thai side. Cambodia’s Constitution provides at Article 30 that in the absence of the King, the President of the Senate assumes the duties of Acting Head of State; Hun Sen is the President of the Senate. The directive issued from the constitutional office that exercises executive authority during the King’s absence. It closed the bilateral track Sihasak had described as a precondition before the conciliation request Cambodia had already filed could be acted upon.
The precedent for Annex V Section 2 compulsory conciliation involving a state with an Article 298 exclusion declaration is Timor-Leste v. Australia (PCA Case 2016-10). Australia had filed an Article 298 declaration covering sea boundary delimitation, the category at issue both in that dispute and in this one. Timor-Leste activated the proviso. The Permanent Court of Arbitration administered the proceedings. The commission’s report was non-binding. UNCLOS Annex V Article 12 provides that the failure of a party to reply to the notification of institution or to submit to the proceedings does not bar the proceedings themselves. Under Annex V Article 3, if a party does not appoint its conciliators within the prescribed period, the Secretary-General of the United Nations makes the appointments. In the Timor-Leste case, the negotiating record the commission produced concluded in a binding Maritime Boundary Treaty signed in March 2018. The precedent is what Sihasak’s “various steps” formulation must contend with, not against.
In November 2024, the Thai government’s Public Relations Department recorded Vice Minister for Foreign Affairs Russ Jalichandra describing MOU 2001 (known in Thailand as MOU 44, after the Buddhist year 2544) as “Thailand’s most effective tool” and stating that “canceling the MOU would not eliminate Cambodia’s territorial claims”. The same statement noted that the cabinet under Abhisit Vejjajiva cancelled MOU 2001 in 2009 and that the cabinet under General Prayut Chan-o-cha reversed the cancellation in 2014. The 2026 cabinet act dismantles what Thailand’s own foreign ministry called its most effective tool. It does not dismantle the claims.
Three questions sit between Cambodia’s notification and the conciliation it requests. The proviso applies to disputes that arise after the Convention’s entry into force on 16 November 1994; Cambodia’s underlying maritime claim line predates that date by more than two decades, though MOU 2001 itself does not. The proviso excludes disputes that “necessarily involve” sovereignty over land territory; the question of Koh Kut has surfaced repeatedly in Thai political discourse around MOU 44, though Thailand’s own government has stated since November 2024 that Koh Kut is not in dispute. The proviso requires that “no agreement within a reasonable period of time” has been reached; twenty-five years of an undelivered framework, terminated by the cabinet act itself, makes that condition the easiest of the three for Cambodia to clear. These are conditional, not foregone. Cambodia’s procedural conduct in the May 2026 window has been to position itself to meet them. Thailand’s procedural conduct has been to make political claims the proviso does not recognize and to issue ministerial statements that contradict each other within the same government.
The Thai cabinet has voted. Cambodia’s diplomatic note has been sent. Hun Sen’s directive has been issued in his constitutional capacity as Acting Head of State. Sihasak’s negotiating position has been published in Thai media. Anutin’s Facebook post claims a notification that does not match the form Articles 65 and 67 require. As of 19 May, no instrument signed by the Thai Head of State, Head of Government, or Foreign Minister has been communicated to the Cambodian side.