The subject. Аt the beginning of the 21st century the growing interest of the parties to the dispute and the arbitral tribunals in the rules of customary international law became apparent. This has raised doctrinal and practical questions about the relationship between treaty norms and rules of customary international law in the field of foreign investment protection. The most discussed of them were the issues of filling the gaps in international treaties through the application of the rules of customary international law and the establishment by the arbitral tribunals of the content of the customs they need to apply during dispute resolution.

Materials and methods. This research carried out a scientific analysis of the practice of investor–state arbitral tribunals regarding the interpretation and application of the rules of customary international law in settlement of investment disputes.

Discussion. Numerous investor-state arbitration awards show that arbitration tribunals constantly use the rules of customary international law in dispute resolution. Moreover, arbitrators often refer to the rules of customary international law as a separate legal basis for their conclusions. As modern practice shows, in most cases arbitrators are not inclined to consider the existence or the absence of State practice or opinio juris, instead relying on the conclusions about the existence of a customary rule made earlier by the ICJ, the PCIJ and other arbitration tribunals, as well as international treaties, reports of the International Law Commission and doctrine.

The main results and conclusions. The rules of customary international law are applied by the arbitral tribunals when considering a wide range of issues (jurisdiction, organization of arbitration, applicable substantive law and liability issues). A vivid example of the use of the rules of customary international law in the settlement of investment disputes to fill the gaps in international treaties was a reference by arbitral tribunals to the Articles on Responsibility of States for Internationally Wrongful Acts adopted by the International Law Commission as a codification of existing rules of customary international law in this area.

However, the peculiarity of the use of the Articles by the arbitral tribunals is that the arbitrators consider it as a document containing formulations identical in content to existing rules of customary international law. This leads to the “automatic” application by the arbitrators of the provisions of the Articles to the facts of the case without analysing State practice. Moreover, application of the provisions of the Articles by the arbitral tribunals seems inconsistent, especially in such sensitive issues for the States as the amount of compensation awarded to an investor and contributory fault by an investor. In addition, the high creativity of investors suggests the emergence of claims based solely on the rules of customary international law, and not the provisions of international investment agreements.