This dissertation examines the jurisdictional questions relating to multiparty actions before international tribunals. Multiparty actions are legal claims commenced jointly by multiple claimants.

Jurisdiction over multiparty actions has recently emerged as a contentious issue in investment treaty arbitration (ITA). Tribunals have diverged on – (1) whether multiparty actions require parties’ specific consent to adjudicate claims jointly; and (2) the situations in which claimants can pursue their individual claims jointly in one action. Yet, other international tribunals have regularly exercised jurisdiction over multiparty actions without objection.

In the first part of this study, I explore multiparty actions before international tribunals to distil common approaches that ITA may use. I make three claims – first, international dispute settlement, although conceived to take place bilaterally between the two opposing sides of claimant and respondent, does not inherently exclude multiparty actions. Multiparty actions are bilateral if claimants can organise themselves into one group. The nature of the dispute and the nature of a regime influence when claimants act as one group. Secondly, an international tribunal’s constitutive instrument and procedural rules can be flexibly applied to adjudicate multiparty actions, even in the absence of express procedural provisions. Thirdly, when a tribunal’s jurisdiction depends on a pre-existing jurisdictional title, two factors – the nature of the jurisdictional title and the nature of the dispute – combined explain whether consent exists for multiparty actions. Consent is proved if the pre-existing jurisdictional title binds the disputing parties in a single jurisdictional link. Additionally, the nature of the dispute further influences consent-analysis. A single jurisdictional link suffices to establish jurisdiction when there is one subject-matter in dispute. However, to litigate their distinct claims jointly, claimants must show additional evidence of specific consent, over and above a single jurisdictional link. Yet, even for these cases, it is possible to artificially construct specific consent.

Drawing on this accumulated experience, the second part of this study assesses how investment tribunals should approach their jurisdiction over multiparty ITA. Here, I make two claims. First, through a critical evaluation of existing attempts by tribunals to find consent to multiparty ITA in the text of a State’s arbitration offer in an investment treaty, I contend that consent cannot be derived solely by formalistically applying treaty interpretation principles. Accordingly, I present my second claim that investment tribunals should learn from the approach of other international tribunals and assess consent to multiparty ITA using the dyadic framework of the nature of the jurisdictional title and the nature of the dispute. I argue that the offer-and-acceptance approach to consent in ITA allows the creation of a single jurisdictional link between multiple investors and a host State. This demonstrates consent for multiparty ITA in which the dispute concerns a single investment. For multiparty ITA in which the dispute concerns multiple investments, tribunals can construct consent within the bounds of their authority – as some other international tribunals have done. To that end, I propose the use of judicially-created doctrines to imply consent.