Abaclat v Argentina, the first case in the history of investment arbitration where an investment treaty tribunal dealt with a mass claim (a claim initiated by numerous claimants), stimulated many debates even before the tribunal rendered its decision on jurisdiction and admissibility. This article focuses on only one of the matters on which the case triggered discussion, though probably the most important: whether or not an investment treaty tribunal needs special consent for mass claims. The views of the parties and of the majority and dissenting arbitrators in both Abaclat and Ambiente Ufficio diverged as to whether or not special consent is required for mass claims. The discussion rests mainly upon the qualification of mass claims in investment arbitration and their distinction from class arbitration, traditional mass claims processes and other multi-party proceedings. The article explores the merits of the arguments of both camps to determine which gives a more convincing answer to the question.