This article examines the consequences of the Court of Justice of the European Union’s (CJEU) ruling in Achmea concerning Investor-State Arbitration (ISA) under intra- EU Bilateral Investment Treaties (BITs) from a treaty law perspective. It begins by briefly setting out the arguments of Advocate General Wathelet and the CJEU supporting their different positions on whether intra-EU BITs ISA clauses are compatible with EU law. The article then proceeds to analyse Achmea’s implications for intra-EU BIT ISA. It concludes that, as a result of the CJEU’s ruling, arbitral tribunals are deprived of their jurisdiction to entertain investors’ claims brought under intra-EU BIT ISA clauses. Finally, the article argues that Achmea’s applicability to cases brought under intra-EU BIT ISA clauses is limited, using the application of EU law as a relevant qualification. In order for an arbitral tribunal to be deprived of its jurisdictional competence as a result of Achmea, it must be entitled to interpret and apply EU law directly or indirectly in determining its jurisdiction.

The judgment of the Court of Justice in the Achmea case evoked significant repercussions regarding the application and operation of the bilateral investment treaties (BITs) concluded between EU Member States. As a result of this decision, EU Member States have decided to terminate almost 190 intra-EU BITs. Nevertheless, full implementation of the Achmea judgment remains a complex issue, entangled in political and legal controversies concerning intra-EU BITs which have been present for more than a decade. On a more general level, the implementation process is simultaneously entwined in two other significant debates: the specifics of the rights of investors, and the relationship between EU law and international law.

This article analyses the relationship between the Court of Justice and other international jurisdictions. In particular, it addresses the following question: To what extent is the Court of Justice ready to accept that some aspects of EU law are subject to the jurisdiction of an international body? The answer to this question requires analysis of the precise scope of the principle of autonomy of EU law as this principle could potentially constitute grounds on the basis of which the Court of Justice excludes the transfer of judicial competences to external bodies. For this reason, the article refers to the most important decisions in the field: Opinions 1/91 and 1/92, Opinion 1/09, Opinion 2/13, judgment in C-146/13 Spain v. Parliament and Council and judgment in C-284/14 Achmea. It also discusses the consequences of the application of Article 344 TFEU.