Res judicata, or claim preclusion, is no less important a matter in international arbitration than in national court litigation. Nothing less than the finality of an adjudication is at stake.In principle, a tribunal should decline to adjudicate a dispute if it has already been adjudicated either in a prior judgment or award. Apart from all the disadvantages of duplicative proceedings, a tribunal’s failure to treat a prior adjudication as preclusive made lead to set aside of its award.Even if an adjudication is final and binding, or in the case of an award entitled to recognition, certain so-called “identities” must be established, whether identity of party, identity of claim, or identity of object, or all of the above. But how are they to be defined? Herein lies the difficulty for tribunals.The requirements of res judicata are a matter of forum law and, unlike a national court, an arbitral tribunal has no forum law and derives little if any guidance from the parties’ agreements, arbitration laws or institutional rules. Tribunals have a choice-of-law challenge, their best options being the res judicata norms of one body of national law or another and an international standard if one can be ascertained.

In private international law, the traditional view has been that all aspects of the burden of proof are procedural. It is typically inferred that a forum court properly uses the law of the forum on such matters even when comity dictates the recognition and application of the substantive law of another jurisdiction to the matter in dispute. However, this characterization has never been entirely accurate, at least in American law. Moreover, there has been discernible movement toward the opposite conclusion over the last century. In order to make sense of this, it is necessary to recognize that the two components of the burden of proof, the burden of persuasion and the burden of production, have quite different functions in an adversary system. Once these functions are identified, it becomes clear that only the burden of production, in both its allocation and the severity of the burden that it imposes, should be governed by forum law, while the burden of persuasion, in both its allocation and the severity of the burden that it imposes, should be treated as part of the substantive law that the forum court chooses to apply.