The International Court of Justice, in its 2012 judgmentin the Questions relating to the Obligation to Prosecute or Extradite(Belgium v. Senegal) case, found that Belgium had ius standi toclaim Senegal’s responsibility for the alleged breach of its obligationsunder Articles 6(2) and 7(1) of the Convention Against Tortureand that such claims were admissible. Also, it concluded that it wasnot necessary to determine whether Belgium was ‘specially affected’or ‘injured’. The Court based these findings on the concept of‘obligations erga omnes partes’, which it defined as obligations inthe compliance of which states have an ‘interest’, which, in the caseof the above provisions is a ‘common interest’. Several members ofthe Court rejected the above findings as inconsistent with the lawof international responsibility, and state practice and, for other reasons,ill-grounded. The present paper assesses the Court’s definitionand use of the concept of ‘obligations erga omnes partes’ in lightof public international law. It also analyses the views expressed byother judges sitting on the Bench for the case. The present paper’smain contentions are three. First, the characterisation of obligationsin the performance of which all the states parties to a treaty havean ‘interest’ -arguably a ‘common’ one- as ‘obligations erga omnespartes’ is unnecessary. Secondly, such obligations, as defined, bythe Court, remain merely ‘erga partes’, binding on the parties to thetreaty constituting their source qua parties to the treaty and subject,as any other conventional obligation, to the rule res inter alios actaand to the rules on reservations, which may prevent an ‘obligationerga omnes partes’ from becoming binding on states that have madea reservation to the provision setting out the terms of the obligationand on those accepting such reservations. In this connection, it isdemonstrated that ‘obligations erga omnes partes’ are only of significanceif they are ‘obligations erga omnes’ proper, primarily inthe form of customary obligations under ...