On 16 Jan 2024 GMT+7, Cambodia's National Assembly unanimously resolved to ratify the 1982 United Nations Convention on the Law of the Sea (UNCLOS). This marks a significant step affecting overlapping continental shelf claims (OCA) between Thailand and Cambodia in the Gulf of Thailand, as henceforth the maritime dispute parties will negotiate on an equal international legal basis with greater opportunity to use UNCLOS mechanisms for peaceful dispute resolution.
Cambodia became a party to UNCLOS in July 1983 but delayed ratification for 43 years due to a lack of sufficiently knowledgeable personnel to utilize it effectively. However, international pressure—especially from Japan—and ongoing border disputes with Thailand prompted Cambodia to seek an international forum and mechanisms to help resolve these issues.
Cambodian authorities had prepared for UNCLOS ratification for several years, particularly aware of the conservative right-wing faction in Thailand opposing joint petroleum resource development and maritime dispute resolution under the 2001 Memorandum of Understanding (MOU 44). Multiple international seminars were held in 2024 to assess the pros and cons of ratification, alongside continuous National Assembly committee meetings leading to the unanimous ratification vote.
This ratification strengthens Cambodia's standing on the international stage by establishing a firm international legal basis to protect or contest its rights. It is significant for the maritime dispute with Thailand, which ratified UNCLOS in 2011, because Cambodia can now negotiate, contest rights, or propose solutions within the same legal framework and standards as Thailand.
Strategically, this demonstrates Cambodia's pursuit of an internationally recognized forum and mechanisms accepted as capable of peacefully resolving maritime disputes. Meanwhile, Thailand actively enhances its maritime power to defend sovereignty and marine interests. Cambodia, with less naval strength, chooses international law and mechanisms as guarantees of its security and interests.
Thailand and Cambodia have had maritime disputes due to overlapping continental shelf claims in the Gulf of Thailand since 1972-1973. Although both countries agreed on the 2001 Memorandum of Understanding regarding overlapping continental shelf claims as a framework for maritime delimitation and joint petroleum resource development, political factors within Thailand have prevented significant progress.
Thai conservative right-wing leaders, politicians, and civil society groups oppose MOU 44 over concerns that it might allow Cambodia to assert sovereignty over maritime areas, potentially causing Thailand to "lose territory" such as Koh Kut Island. They also argue Cambodia's declared continental shelf claims exceed international law standards and are excessively large, potentially disadvantaging Thailand in resource sharing.
Although these arguments carry limited weight under international law, nationalist sentiment, residual land border disputes that have led to military clashes, and accusations of overlapping political and energy sector interests have caused many Thai governments to hesitate in advancing negotiations over overlapping maritime claims, resulting in ongoing stagnation.
Nonetheless, Cambodia's UNCLOS ratification is not a panacea resolving all maritime disputes with Thailand. However, the 320 articles and nine annexes of UNCLOS provide useful tools and mechanisms for both countries to seek joint, principled, and equitable solutions as follows.
First, UNCLOS provides a comprehensive legal basis for delimiting exclusive economic zones and continental shelves in Parts V and VI, requiring agreements based on international law as referenced in Article 38 of the International Court of Justice Statute or equitable principles to achieve fair outcomes. If rapid agreement is not possible, UNCLOS allows "provisional measures" that are practicable and do not affect existing rights—indeed, MOU 44's joint development regime is essentially such a provisional measure under UNCLOS.
Second, UNCLOS (Part XV) provides multiple dispute resolution mechanisms, including the International Tribunal for the Law of the Sea, the International Court of Justice, arbitration tribunals established under Annex VII, and special arbitration tribunals under Annex VIII related to specific issues such as fisheries, marine environmental protection, marine scientific research, or navigation and ship-generated pollution.
Article 287 of UNCLOS offers parties the choice of which dispute resolution mechanism to use. Ratifying states involved in disputes may select one or more such tribunals or opt not to select any. If parties agree on a mechanism and no alternative terms exist, the tribunal has automatic jurisdiction over the dispute.
If parties do not agree or choose no tribunal, Annex VII provides for general arbitration designed to prevent legal vacuums. This prevents parties from avoiding dispute resolution by ignoring tribunals. In such cases, either party may request Annex VII arbitration without the other's consent. Parties may appoint individual arbitrators, but if deadlocked, the president of the International Tribunal for the Law of the Sea appoints the arbitrator. The arbitral panel has jurisdiction over its competence and merits. If a party fails to participate or submit pleadings, the tribunal can still proceed and decide, as occurred in the 2016 South China Sea arbitration favoring the Philippines over China.
Applying this to the Thailand-Cambodia dispute raises questions whether Cambodia's UNCLOS ratification compels Thailand to use dispute resolution mechanisms under Article 287 or, if Thailand refuses, triggers automatic Annex VII arbitration.
The answer is no. Although Thailand ratified UNCLOS earlier, it has invoked Article 298 to declare reservations excluding compulsory jurisdiction over disputes involving UNCLOS interpretation, maritime delimitation, and military activities.
In summary, Cambodia's UNCLOS ratification will have greater political than legal impact on Thailand. Legally, Cambodia cannot readily compel Thailand to appear before UNCLOS tribunals. Strategically, however, Thailand faces three layers of pressure as follows.
First is narrative risk: Cambodia's claim to abide by international rules appears stronger, while Thailand risks being seen internationally as obstructive and politically motivated, resulting in reputational damage even if no legal case is lost.
Second is negotiation risk: Cambodia gains leverage to pressure Thailand to return to UNCLOS principles such as equitable solutions and provisional measures, making joint development proposals appear more reasonable.
Third is economic and investment risk: With UNCLOS as a clear framework, investors and energy companies will be more cautious of reputational and international dispute risks. Thailand's previous reluctance to engage out of fear of negotiation disadvantages will face pressure to clarify and structure rules regarding overlapping claims based on international standards.
At minimum, Thailand must address private concessionaires operating in overlapping claim areas: with increased international mechanisms and tools, will Thailand continue to avoid clarifying the dispute's resolution? How long will they make companies wait and how much business opportunity will be lost?
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