Résumé : Abstract The entry into force, on January 1st , 2012, of the European Union Directive 2008/101/EC extending the European Emission Trading System to domestic and international civil aviation has taken the dispute regarding its legitimacy to unprecedented heights.The choice of the EU legislator to include foreign air carriers and their CO2 emissions that occurred beyond EU airspace infuriated third countries, while the fact that the directive applies the same treatment to all airline operators whatever their nationality met vivid criticism from developing countries, in particular China and India. This paper investigates why the environmental objectivepursued by the EU Aviation ETS cannot make up for its unilateral adoption, despite staging multilateral negotiations and despite the flourishing national transplants of the ETS system in other jurisdictions, and provides a preliminary assessment of what the current row implies for the global governance of climate change.Devoting particular attention to the positions of the EU and China in this dispute, it argues that the opposition to EU endeavour finds its roots in the normative frictions between the climate change regime and the international aviation regime, while the lack of process legitimacy of EU unilateralism provoked third countries’ claims to the infringement of their national sovereignty. Thus, it concludes that in the current international system, the harmonization of regimes’ normative goals and principles must result from a political choice, the absence of whichcan effectively frustrate the achievement of multilateral cooperation goals.Moreover, in such context, the unilateral imposition of an alternative path involving the other regime members against their express will, to palliate multilateral norm-making, is likely to meet increasingly strong opposition from an increasing number of powerful countries.