The 2011 decision on jurisdiction and admissibility in Abaclat and Others v Argentina has started a discussion about mass claims processes in investment treaty arbitration. The tribunal concluded that although proceedings were initiated in aggregate, the continuance of the case contained a representative feature. This determination led them to declare that the applicable procedure could and had to be adapted. Today, the legacy of Abaclat and the availability of mass claims procedural devices in investment treaty arbitration remain questionable: can mass claims investment arbitration be qualified as ‘class-like’? If so, does it satisfy the fundamental principles of arbitration (particularly the principle of consent)? This article takes a comparative approach to answering these questions by putting mass claims investment arbitration procedures and United States class actions processes side-by-side. It argues that mass claims arbitration as construed in Abaclat cannot satisfy fundamental arbitration principles because it fails to observe the inextricable link between the parties’ consent, representative procedure, and representative relief. It is therefore wrong to view mass claims arbitration as an available device for investors in investment treaty arbitration.