The procedural rights of taxpayers often have a significant impact on how certain rights provided by substantial tax rules can be exercised. Constitutional courts have the jurisdiction to review whether such procedural rules infringe any constitutional rights, whereas the ECJ has jurisdiction to review whether the application of procedural rules unlawfully limit the application of rights granted by EU law. In this article, the author discusses a Hungarian case in which the decisions of the two courts seemingly clashed and analyses what the potential implications of this could be.