Further to the SECIL judgment (C-464/14) delivered by the European Court of Justice (ECJ) in November 2016, this article explores how Association Agreements (AAs) concluded by the European Union (EU) with third countries connect with EU primary law, especially the Treaty on the Functioning of the European Union (TFEU), and discusses their possible implications in tax matters. It investigates the consequences of the variations in purpose and wording of such international instruments and identifies opportunities, uncertainties, shortcomings and potentialities deriving therefrom.