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.