• Tutorial video


Mr Alexey Drobyshev

Senior Legal Counsel - JSC Biocad


Mr Antolín Fernández Antuña

Arbitrator & Counsel, Managing Partner - Antuña & Partners

See all

Expert Witness

I. Definition


The role of an expert witness (or simply an “expert”) is to opine on specialist matters beyond the expertise of the tribunal.1 Disciplines requiring expert input are usually legal, technical/scientific, and valuation/quantum.2

II. Related Wiki Notes

III. Legal authority


Arbitration rules (ICSID Arbitration Rules, ICSID Additional Facility Rules, UNCITRAL Arbitration Rules) have no detailed provisions concerning expert evidence. The IBA Rules on the Taking of Evidence in International Arbitration are commonly used,3 sometimes as a nonbinding reference or guidance.4 A less commonly used guidance document is the CIArb Protocol for the Use of Party-Appointed Expert Witnesses in International Arbitration. Certain investment treaties authorize a tribunal to appoint an expert,5 sometimes with a qualifier that the parties do not oppose (sometimes jointly) the appointment,6 as did NAFTA.7

IV. Party-appointed and Tribunal-appointed experts


An expert can be appointed by a party or by a tribunal.8 In investment arbitration, party-appointed experts are more common.9 However, sometimes a tribunal will exercise its power to appoint an expert.10

V. Independence, impartiality, transparency


Experts and their evidence should be objective.11


The IBA Rules set out to achieve expert objectivity by requiring the expert to disclose his or her instructions, state that the expert opinion reflects that expert’s genuine belief, and stating his or her independence.12 See further Disqualification of expert witness


A tribunal-appointed expert has to be independent from the parties, their legal advisors and the arbitral tribunal.13 Lack of independence of an expert could affect the right of an arbitrator to hear the case, but does not do so automatically.14 See further Conflicts of interest, Arbitrator impartiality and independence and Disqualification of arbitrators

VI. Experts on applicable law


Expert evidence on both domestic law (usually of the host State)15 and international law is common in investment arbitration. Tribunals also accept expert evidence on the Applicable Law, whether domestic or international (see Applicable Law), in addition to the legal pleadings of the parties’ counsel.16 This is open to criticism because an expert submission may be conflated with the legal submissions of the counsel, which is inconsistent with general international law approach of separate roles of experts and counsel.17

VII. Joint statements


Party-appointed experts of like disciplines may be required to meet with each other in advance of the hearing and prepare a joint statement of matters agreed and disagreed.18 This technique serves to narrow the areas of disagreement between the experts and maximise the compatibility between their evidence,19 making it easier to process for the tribunal and the parties.20

VIII. Witness conferencing


Witness conferencing, or “hot-tubbing”, is a questioning of several experts of like disciplines at the same time, putting them in confrontation.21 This tool can be used to streamline the giving of expert evidence at the hearing by allowing the experts to directly challenge each other’s views at the hearing.22


CIArb Guidelines on Witness Conferencing in International Arbitration provide guidance on how to determine whether to deploy hot-tubbing and if so, how to design the procedure in a given arbitration.

IX. Post-hearing involvement


In the arbitrations that allow post-hearing submissions, experts can be further involved, particularly for assessment and re-examination of the parties’ economic models for damages calculations.23 An expert’s testimony can be subjected to another round of review by the other party’s expert so that that party’s counsel are better able to identify the weaknesses in the expert evidence of the other side.24

X. Previously published writings


As a practical matter, previous published writings should be checked for consistency with his or her expert opinion. Inconsistencies have no procedural consequences, but they may detract from the weight of the evidence.25 See further Disqualification of expert witness


Ashford, P., Article 5 – Party-Appointed Experts, in Ashford P., The IBA Rules on the Taking of Evidence in International Arbitration, pp. 105-120.

Ashford, P., Article 6 – Tribunal-Appointed Experts, in Ashford P., The IBA Rules on the Taking of Evidence in International Arbitration, pp. 121-127.

Blackaby, N. and Wilbraham, A., Practical Issues Relating to the Use of Expert Evidence in Investment Treaty Arbitration, ICSID Review, 2016, pp. 655-669;

Cremades, B., The Expert Witness in International Arbitration, in Betancourt, J.C. (ed.), Defining Issues in International Arbitration: Celebrating 100 Years of the Chartered Institute of Abritrators, p. 192.

Dragiev, D., Rationalizing Applicable Law in Investor-State Disputes in Absence of Express Choice of Law Under Article 42 (1) of ICSID Convention – Part II, Kluwer Arbitration Blog.

Ehle, B., Practical Aspects of Using Expert Evidence in International Arbitration, in Roth, M.and Geistlinger, M. (eds.), Yearbook on International Arbitration, Vol. II, 2012, pp. 75-84.

Foster, C., The Role of Adjudicators and the Role of Experts, in Foster, C., Science and Precautionary Principle in International Courts and Tribunals, 2011, pp. 126-182.

Khodykin, R. and Mulcahy, C., Article 5: Party-Appointed Experts, in Fletcher, N. (ed.), A Guide to the IBA Rules on the Taking of Evidence in International Arbitration, 2019, pp 279-322. 

Khodykin, R. and Mulcahy, C., Article 6: Tribunal-Appointed Experts, in Fletcher N. (ed.), A Guide to the IBA Rules on the Taking of Evidence in International Arbitration, 2019, pp 323-360.

Singh, K., Chandran, S., Premkumar, S. and Foo, A., The “Additional Weapon”: Practical Tips for Effective Expert Conferencing in Arbitration, Kluwer Arbitration Blog.

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