Double-hatting in the Investor-State Dispute Settlement system: multifunctionality vs. strict specialization

The subject. In modern conditions, the problem of independence and impartiality of arbitrators within the decentralized, fragmented and largely ad hoc Investor-State Dispute Settlement (ISDS) system manifests itself in three main forms: issue conflict, repeat appointments, and double hatting. This article focuses on the latter phenomenon.

Methods. The research is based on traditional methods of system analysis, deduction and induction, as well as the study of international legal instruments, arbitral decisions and national court practice.

Main results. Investment arbitration, having inherited key features from international commercial arbitration – particularly the parties’ right to appoint arbitrators – reveals specific complexities. Arbitrators are typically not full-time adjudicators and often combine this role with other professional activities. On the one hand, such duality ensures the availability of highly specialized expertise. On the other hand, it creates risks, as the only formal requirement for appointment is the parties’ choice. The widespread practice of double hatting – simultaneously or sequentially acting as arbitrator, counsel, or expert – may undermine the principles of independence and impartiality, and ultimately the rule of law. Against this background, current ISDS reform discussions, including proposals for a permanent investment court, aim to reduce or eliminate role combination. However, the justification and usefulness of such reforms remain debatable.

Conclusions. States are increasingly focused on strengthening safeguards of arbitrators’ independence and impartiality. This includes limiting double hatting, tightening eligibility criteria, refining appointment and challenge procedures, and enshrinement in international law the duty of arbitrators to disclose relevant circumstances or decline appointments. At the same time, a new form of “moral capital” is emerging, reflected in the fact that arbitrators voluntarily renounce performing multiple roles.

1. Simões F.D. Hold on to Your Hat! Issue Conflicts in the Investment Court System. The Law and Practice of International Courts and Tribunals, 2018, vol. 17, iss. 1, pp. 98–116. DOI: 10.1163/15718034-12341370.

2. Reinisch A., Knahr C. Conflict of Interest in International Investment Arbitration, in: Peters A., Handschin L. (eds.). Conflict of Interest in Global, Public and Corporate Governance, Cambridge University Press, 2012, pp. 103 124.

3. Langford M., Runar D.B., Lie H. The Revolving Door in International Investment Arbitration. Journal of International Economic Law, 2017, vol. 20, no. 2, pp. 301–332.

4. Sucharitkul V. The Ban on Double Hatting in Investment Arbitration: A Double-Edged Sword?. National Law School Business Law Review, 2021, vol. 7, iss. 2, pp. 44–72.

5. Berzero R., Horvath G.J. Arbitrator and Counsel: the Double-Hat Dilemma. Transnational Dispute Manage ment, 2013, no. 4: Ten Years of Transnational Dispute Management, available at: https://www.transnational-dispute-management.com/article.asp?key=1985 (accessed: December 1, 2025).

6. Tanwar M.K. Addressing the Inadequacies: A New Multi-Faceted Solution to Double Hatting in ISDS. LSE Law Review, 2021, vol. 7, no. 1, pp. 115–165.

7. Burgos M.A. Double-hatting in international commercial arbitration?, in: González-Bueno C. (ed.). 40 under 40 International Arbitration (2018), Madrid, Dykinson Publ., 2018, pp. 87–98.

8. Lévesque C. Canada’s Pro-Ban Stance on Double-Hatting: Playing the Long Game in ISDS Reform?. Canadian Yearbook of International Law, 2021, vol. 58, pp. 382–407.

9. Schiappa-Pietra G., Fuentes J., Mejía D. Pasando el sombrero: una revisión del double-hatting desde la experiencia internacional y su influencia en la concentración de funciones dentro del circuito arbitral peruano. Advocatus, 2023, no. 43, pp. 335–346. (In Spanish).

10. Langford M. The Changing Sociology of the Investment Arbitration Market: The Case of Double Hatting, in: Bull C., Malintoppi L., Partasides C. (eds.). Arbitration’s Age of Enlightenment?, Kluwer Law International Publ., 2023, pp. 591–621.

11. Costa J.A.F. Comparing WTO Panelists and ICSID Arbitrators: The Creation of International Legal Fields. Oñati Socio-Legal Series, 2011, vol. 1, no. 4: Socio-Legal Aspects of Adjudication of International Economic Disputes, available at: https://opo.iisj.net/index.php/osls/article/view/63.

12. Ginsburg T. The Culture of Arbitration. Vanderbilt Journal of Transnational Law, 2003, vol. 3, iss. 4, pp. 1335–1345.

13. Ispolinov A.S. International Investment Law and Investment Dispute Resolution Mechanisms in the 21st Century, in: Prognozy razvitiya mezhdunarodnogo ekonomicheskogo prava i mezhdunarodnogo pravoporyadka v XXI veke, Collective Monograph, Moscow, All-Russian Academy of Foreign Trade Publ., 2025, pp. 94–110. (In Russ.).

14. Giorgetti C. The Draft Code of Conduct for Adjudicators in Investor – State Dispute Settlement: A Low hanging Fruit in the ISDS Reform Process. Journal of International Dispute Settlement, 2023, vol. 14, no. 2, pp. 176 191.

15. Sands P. Conflict and Conflicts in Investment Treaty Arbitration: Ethical Standards for Counsel, in: Rovine A. (ed.). Contemporary Issues in International Arbitration and Mediation: The Fordham Papers, Brill Publ., 2012, pp. 28 49.

16. Devarakonda Y. Restricting Double-Hatting to Safeguard International Arbitrations. Pepperdine Dispute Resolution Law Journal, 2022, vol. 22, no. 1, pp. 65–94.

17. Fach Gómez K. The Duty of Disclosure: An Overview, in: Fach Gómez K. Key Duties of International Investment Arbitrators: a Transnational Study of Legal and Ethical Dilemmas, Springer Publ., 2019, pp. 25–77. DOI: 10.1007/978-3-319-98128-4_2.

18. Kiseleva O. The Practice of Self-Recusal and Recusal of Judges of the International Court of Justice. Zhurnal VShE po mezhdunarodnomu pravu = HSE University Journal of International Law, 2024, vol. 2, no. 3, pp. 42–51. DOI: 10.17323/jil.2024.23987. (In Russ.).

19. Vantrees A. A Review of “Moonlighting” ICJ Judges and the International Call for Impartiality in Investor State Dispute Resolution. World Arbitration and Mediation Review, 2018, vol. 12, no. 4, available at: URL: https://jusmundi.com/en/document/publication/en-a-review-of-moonlighting-icj-judges-and-the-international-call-for-impartiality-in-investor-state-dispute-resolution (accessed: December 1, 2025).

20. Giemza W. Moonlighting Revisited: International Court of Justice Judges as Adjudicators in Investment Arbitration. African Journal of International Economic Law, 2024, vol. 4, pp. 4–28.

21. Ispolinov A.S. Where is the present investment arbitration heading toward?. Rossiiskii yuridicheskii zhur nal = Russian juridical journal, 2015, no. 3, pp. 80–96. (In Russ.).

22. Titi C. Who’s Afraid of Reform? Beware the Risk of Fragmentation. AJIL Unbound, 2018, vol. 112, pp. 232 236.

23. Pimenova S. Investment arbitration reform in the eyes of the European Union: problems and perspectives. Mezhdunarodnoe pravosudie = International justice, 2018, no. 4 (28), pp. 113–125. (In Russ.).

24. Sardinha E. The New EU-Led Approach to Investor-State Arbitration: The Investment Tribunal System in the Comprehensive Economic Trade Agreement (CETA) and the EU–Vietnam Free Trade Agreement. ICSID Review – Foreign Investment Law Journal, 2017, vol. 32, iss. 3, pp. 625–672.