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M. William Ahern

Associate in International Arbitration - Mayer Brown

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Admissibility (Evidence)

I. Definition and context


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. 

II. Applicable rules


The wide discretion available to arbitrators is confirmed by rules commonly applicable in investment arbitrations. Article 34(1) of the ICSID Arbitration Rules provides that the [t]he Tribunal shall be the judge of the admissibility of any evidence adduced and of its probative value." 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 Article 27(4) of the UNCITRAL Rules (2013) provides equally broad discretion to arbitrators to determine admissibility.3 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,4 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.

III. Examples of specific issues of admissibility that arise

A. Admissibility of illegally obtained evidence


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.5 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.6 The tribunal also took into consideration the marginal relevance and lack of materiality of the evidence in question.7 


In Libananco, the Tribunal prohibited entry to the record of thousands of privileged and/ or confidential e-mails between the investors and its counsel, obtained through surveillance, citing the Parties' obligation to arbitrate fairly and in good faith. Another line of decisions involved the submission of WikiLeaks cables into evidence, with tribunals failing to address the admissibility of such evidence,8 save for the dissenting arbitrator in ConocoPhillips, who found that the materiality of such evidence warranted its conclusion.9 The tribunal in Caratube, on the other hand, reportedly10 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.11

B. Admissibility of evidence obtained by the State through its police powers


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.12 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.13 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.14


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.15


Chester Brown, C., A Common Law of International Adjudication, 2007.

Dany 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.

Sourgens, F.G., Duggal, K. and Laird, I.A., Evidence in International Investment Arbitration, 2018.

Niyungeko, G., La Preuve Devant les Juridictions Internationales, 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., Havalic, M., Fruits of the Poisonous Tree: The Admissibility of Unlawfully Obtained Evidence in International Arbitration, Transnational Dispute Management, 2015.

Wiebecke, M., Evidence and Proof, The Investment Treaty Arbitration Review, 5th ed., 2020.

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