The near-death of the EU-Ukraine Association Agreement and CETA painfully illustrated that the conclusion mixed agreements, i.e. agreements that list the EU, its Member States and a third party as contractors, may be derailed by a negative vote of (sub-)national decision-makers. Such a non-ratification entails a problematic conundrum: Despite the requirement for national ratification under international law, a Member State violates the EU’s legal principles of conferral and loyal cooperation when vetoing a mixed treaty in its entirety. The present paper therefore argues that the Member States are not competent to reject the EU exclusive parts of a mixed treaty in their own right. It suggests that the EU’s and the Member States’ legal authority to ratify mixed agreements is contingent on who owns and who exercises treaty-making power for substantive components and outlines several practical ways to align national (non-)ratification with the EU’s law on competences and procedure.