Admissibility (Evidence)

Evidence must be admitted to the record of arbitral proceedings before its probative value can be weighed and applied. While practices may differ depending on the legal background of participants in the arbitral process, strict evidentiary rules typical in domestic systems generally do not apply in international arbitration (including investment arbitration).¹ Arbitrators enjoy broad discretion on evidentiary issues, including with respect to determining whether a specific piece of evidence is to be admitted or excluded.

The wide discretion available to arbitrators is confirmed by rules commonly applicable in investment arbitrations. Rule 36(1) of the ICSID Arbitration Rules provides that "The Tribunal shall determine the admissibility and probative value of the evidence adduced." Tribunals have relied on this as well as other provisions to confirm the absence of technical or rigid rules to judge admissibility of evidence in the ICSID context.² It also offers arbitrators wide discretion in determining whether the question of admissibility of evidence is a procedural or substantial matter.³ 

The tribunal’s discretion with regards to the admissibility of evidence may not be re-determined by ad hoc Committees, given that ICSID annulment proceedings are not appeals.⁴ See further Annulment of ICSID awards, Section III. As such, the tribunal’s evidentiary discretion may not be challenged in itself, but the exercise of this power may warrant the annulment of the award (for instance if it constitutes a breach of a fundamental rule of procedure such as the right to be heard).⁵

Article 27(4) of the UNCITRAL Rules (2010) provides equally broad discretion to arbitrators to determine the admissibility of evidence.⁶

Arbitrators in investment arbitrations also regularly turn to the IBA Rules on the Taking of Evidence in International Arbitration for guidance, including with respect to the admissibility of evidence,⁷ Article 9(1) of those Rules also provide broad discretion to arbitrators on questions of admissibility of evidence, while Article 9(2) sets outs specific grounds on which evidence can be excluded, such a legal impediment or privilege and political or institutional sensitivity.⁸ However the threshold to exclude evidence seems high since the arbitral tribunal must ensure fair proceedings.⁹ See further Evidence in Investor-State Arbitration, Section II and Standard of Proof, Section II. 

“[T]he right of parties […] to seek advice and to advance their respective cases freely and without interference” has been understood to be a fundamental aspect of the arbitral process.¹⁰ To this extent, attorney-client privilege can be used to render specific communications and documents exchanged between attorneys and their clients (or experts or other third parties) inadmissible.¹¹ However, the privilege must not have been waived¹² and must be justified meaning that the attorney (or the agent of the attorney¹³ or the government attorney¹⁴ or the in-house counsel¹⁵) must be providing legal advice.¹⁶

No approach has found uniform application by investment arbitration tribunals when considering admissibility of illegally obtained evidence.

In refusing to admit certain documents to the record, the tribunal in Methanex found that those documents had been obtained unlawfully and admitting them would breach a duty of good faith, under the applicable UNCITRAL Rules as well as generally in international arbitration.¹⁷ In determining whether the documents had indeed been obtained unlawfully, the tribunal noted that prima facie evidence of unlawfulness had been provided by the United States – thus shifting the burden of proof to Methanex.¹⁸ The tribunal also took into consideration the marginal relevance and lack of materiality of the evidence in question.¹⁹

Another line of decisions involved the submission of WikiLeaks cables into evidence, with tribunals failing to address the admissibility of such evidence,²⁰ save for the dissenting arbitrator in ConocoPhillips, who found that the materiality of such evidence warranted its inclusion.²¹ The tribunal in Caratube, on the other hand, reportedly²² did address whether evidence taken from a website containing hacked government documents could be admitted. In allowing the submission of such evidence, it was noted that the documents were already legally publicly available and were not privileged.²³

Similarly, the Dominicana Renovables tribunal, while noting that recordings of a conversation made without one of the protagonists’ knowledge raised questions of transparency and good faith, refused to rule on the legality of the recording and decided not to exclude it as evidence, underlining the nature of the conversation which was neither private nor confidential.²⁴ 

The new 2020 version of the IBA Rules on the Taking of Evidence in International Arbitration now provides in Article 9(3) that an arbitral tribunal “may, at the request of a Party or on its own motion, exclude evidence obtained illegally.”

The Institut de Droit International has recognized tribunals' inherent power to maintain equality of the parties by ensuring States do not obtain an unfair advantage in obtaining evidence through the use of police powers, including by requiring States to make an application before deploying evidence obtained during a criminal investigation.²⁵ In Awdi v Romania, the Claimants sought to have an illegality objection ruled inadmissible, including on the basis that it was based on evidence obtained through ongoing criminal investigations, thus breaching the principles of presumption of innocence as well as equality of arms.²⁶ In rejecting the application, the tribunal found that the real issue raised was the weight and probative value to be given to the evidence, rather than its admissibility, and that it would be guided by the presumption of innocence rule when assessing that evidence.²⁷ 

The tribunal in Glencore took a different approach, excluding documents from the record that had been seized from the premises of one of the claimant companies in the context of a preliminary investigation, on the basis that this breached the principles of good faith and equality of arms.²⁸ Similarly in Libananco, the Tribunal prohibited the entry into the record of thousands of privileged and confidential e-mails between the investors and its counsel, obtained through surveillance during criminal investigations, citing the Parties' obligation to arbitrate fairly and in good faith.²⁹

A tribunal can limit the disclosure of evidence to ensure the protection of national concerns such as the secrecy of criminal investigations.³⁰

The tribunal may also exclude evidence to protect confidential information³¹ such as private records³² and commercial or trade secrets.³³ Confidentiality orders can also be imposed to ensure that parties do not use their opponent’s confidential information for purposes other than the arbitration proceedings.³⁴ 

Although usually admissible in international arbitration, hearsay (or double hearsay) evidence usually requires confirmatory evidence.³⁵

Evidence generated after the dispute has arisen (the “critical date”) may be admissible in two situations: (1) where it merely confirms pre-critical date evidence, and (2) in the absence of conclusive pre-critical date evidence, with the caveat that special attention is to be given to the proper weight to attach to such evidence.³⁶ 

Although in principle open to receiving new evidence during the arbitral proceedings,³⁷ tribunals often require exceptional circumstances (i.e. “relevant to the dispute and material to its outcome”) to admit new evidence into the record once the last written submission has been filed.³⁸ Where such circumstances are met and new evidence is introduced, the tribunal will allow the opposing party to rebut the additional evidence.³⁹

It has been held that the late introduction of documents such as slides or visual aids that are limited to the illustration of previously heard evidence do not qualify as new evidence.⁴⁰

The closing of the proceedings prevents the parties from submitting new evidence. However, the proceedings might be reopened on the basis of new evidence. See further: Closing of the proceedings. 

In principle, ICSID ad hoc Committees⁴¹ as well as national courts applying domestic legislation (notably based on the UNCTRAL Model Law)⁴² do not allow for the admission of new evidence during annulment or set-aside proceedings. However, it may be allowed under very specific circumstances, including inter alia when the evidence could not have been obtained earlier using reasonable diligence⁴³ and if it is relevant to the ground invoked for annulment or setting aside of the award.⁴⁴ 

Brown, C., A Common Law of International Adjudication, Oxford University Press, 2007.

Khayat, D. and Ahern, W., Allegations of Illegality in Investor-State Arbitration and the Presumption of Innocence, Indian Journal of Arbitration Law, Vol. 6, Issue 1, 2017.

Sourgents, F.G., Duggal, K. and Laird, I., Evidence in International Investment Arbitration, Oxford University Press, 2018.

Niyungeko, G., La Preuve Devant les Juridictions Internationales, Bruylant, Éditions de l’Université de Bruxelles, 2005.

Ireton, J.O., The Admissibility of Evidence in ICSID Arbitration: Considering the Validity of WikiLeaks Cables as Evidence, ICSID Review, Vol. 30, Issue 1, 2015.

Boykin, J.H. and Havalic, M., Fruits of the Poisonous Tree: The Admissibility of Unlawfully Obtained Evidence in International Arbitration; Transnational Dispute Management 5, 2015.

Wiebecke, M., Chapter 9: Evidence and Proof, in Legum, B. (ed.), The Investment Treaty Arbitration Review, The Law Reviews, 5^th ed., 2020.