The duty to disclose is the main mechanism for ensuring the arbitrator’s integrity and promoting the legitimacy of international arbitration. Little attention has been paid to the parties’ duty to investigate, which has contributed to the adoption of malicious tactics and opportunistic inertia by the parties at the beginning of the proceedings. This paper analyses whether the parties have a duty to investigate in international arbitrations seated in England, France and Switzerland, and suggests some parameters to identify what information should be investigated by them. Literature review and court decisions’ analysis through inductive methodology revealed, firstly, that the greater the importance ascribed to the duty to disclose in a given legal system, the narrower the scope of the duty to investigate, and, secondly, that civil law jurisdictions are more likely to recognise the existence of a duty to investigate than common law jurisdictions. Considering the diversity of approaches in different jurisdictions and given the importance of the duty to investigate in promoting the legitimacy of international arbitration, it would be recommended that institutional rules or the IBA Guidelines stipulated it. However, institutional rules usually do not impose such a duty on the parties, and the IBA Guidelines, recently updated in its latest 2024 version, impose a narrow duty to investigate on them, restricted to information which they must disclose regarding possible conflicts of interest with the arbitrator. The paper concludes that parties should have a broader duty to investigate, encompassing reasonably accessible information related to the arbitrator, and proposes that, to ascertain whether the parties fulfilled such duty, their conduct should be compared with the standard of diligent conduct, which establishes the level of effort in investigating information that should be expected from the party in each situation.

1. Introduction. 2. Ascertaining the independence and impartiality of the arbitrator. 2.1. The importance of the independence and impartiality of the arbitrator. 2.2. The content of the arbitrator’s duty to disclose. 3. Widening the focus: imposing a duty to investigate on the parties. 3.1. The parties’ duty to inform. 3.2. Why the parties should have broader duties in ascertaining the independence and impartiality of the arbitrator. 3.3. Investigating the existence of a duty to investigate in arbitrations seated in England, France, and Switzerland. 4. Towards the recognition of a duty to investigate in international commercial arbitration: a proposition. 5. Conclusion. 6. Bibliography.