Given the twin goals of finality and efficiency, the doctrine of res judicata has come to be applied, although less frequently, in the international arbitration context as well. However, being largely perceived as a proverbial ‘twilight issue’ in international arbitration, its application is fraught with uncertainties and inconsistencies. Amongst the more compelling concerns regarding the subject matter, this Article tackles the ambiguities around the choice of law analysis for preclusion standards; the doubts regarding the arbitral tribunal’s kompetenz-kompetenz to address the issue; and the peculiar nature of jurisdictional court decisions and its res judicata effect in subsequent arbitral proceedings. Rarely, if ever, does the lex arbitri shed light on the precise standards of preclusion to be applied in a particular case. Instead, the choice of law analysis by arbitral tribunals are guided by a fluid balancing act between varying degrees of private rights and public interests. While the The International Law Association (ILA) Recommendations (Resolution No. 1/2006) do come close to a purported international standard, its limited acceptability within the community and lean adoptability across jurisdictions, brings to the fore the uncertainties attached to the doctrine itself. Confusion further ensues when the authority of the tribunal to decide on its own jurisdiction is brought into question on confronting the defense of res judicata. While the New York Convention’s mandate of recognition of awards empowers the Courts to afford res judicata effect to a prior adjudication, the same conflicts with the arbitral tribunal’s own competence to address arguably procedural arbitrability issues such as this. These concerns amplify manifold when an arbitral tribunal encounters a prior Court’s decision regarding the tribunal’s jurisdiction, including the question of non-arbitrability of the disputed claim. In the absence of exacting standards and principles to deal with any of these issues, different tribunals have been discharging their own brand of the doctrine’s broad interpretation. This Article expounds the existing literature on the subject, and thereafter, attempts to analyse each of these complex and controversial issues to better equip practitioners and arbitrators when faced with such concerns; at least until universal conformity is achieved through promulgations bordering a truly international standard. Res Judicata, Claim Preclusion, Choice-of-Law, Kompetenz-Kompetenz, Jurisdictional Decisions