From Annulment to Appeal: Worlds Apart or Best of Both Worlds?

Current efforts to modernise investor-state dispute settlement (ISDS) commonly envisage the establishment of an appellate mechanism, marking a departure from the traditional preference for annulment as the principal remedy to challenge arbitral awards. This article analyses the differences and similarities between the mechanisms of annulment and appeal, notably in respect of their function, scope of review, and available outcomes. It then assesses the combined approach seen in recent proposals for a future appellate mechanism, which include not only common grounds of appeal that target errors of fact or law, but also classic annulment grounds that focus on issues of jurisdiction and procedure. The article concludes that annulment grounds can continue to fulfil a useful function alongside grounds of appeal, by ensuring not only the substantive correctness of the decisions under review but also the procedural legitimacy of the first-instance proceedings.