This paper analyses case law on Article 52(1) (d) of the ICSID Convention, according to which an award rendered by an ICSID tribunal can be annulled by an ad hoc Committee on the grounds of a serious departure from a fundamental rule of procedure. To be able to successfully invoke this ground before an ad hoc Committee, the applicant has to demonstrate both that the departure from a rule of procedure is serious and the rule fundamental. These requirements are cumulative. The decisions on annulment which were examined in this paper illustrate that ad hoc Committees interpret narrowly and with great care this ground of annulment because it is related to the principle of due process. An important hurdle to any request for annulment is the wide discretion which arbitral tribunals enjoy in the way they conduct the evidentiary proceedings. The ad hoc Committee in the Azurix case held that “it is only where the exercise of that discretion, in all the circumstances of the case, amounts to a serious departure from another rule of a fundamental nature that there will be grounds for annulment under Article 52(1)(d)”. This discretion as well as the professionalism and expertise of the arbitrators sitting in ICSID panels will continue to be the major obstacles for annulment requests on the basis of Article 52(1)(d).