This article examines the impact of the principle of effectiveness of EU law on private arbitration. It uses the frame of post-award litigation to demonstrate that the relationship between these two normative orders is transversal and potentially very disruptive. This is evidenced by the alteration of the burden of proof in post-award actions, the irrelevance of the loss of the right to object, the widening of judicial review over awards that violate EU public policy, and the possibility to scrutinize errors of EU mandatory law concerning the merits of the case. The result is the weakening of the finality of awards and the replacement of the principle of procedural autonomy of Member States by European procedural primacy.