The debate about the legitimacy of the EU and the possibilities for its democratization has addressed so far only rarely the question of the role of the media. An instrumental approach prevails towards the media, acknowledging that the so-called gap between the EU and its citizens is grounded in a communication deficit and that the EU should therefore strive towards a higher legitimacy in terms of public accountability, openness and participation, in other words of democracy. The article discusses these technical aspects of 'public-sphere building from above' in relation to the systematic constraints on mediatization that result from the inertia of the existing (national) media spheres. On the basis of this, an alternative understanding of mediatization and its ambivalent effects on the legitimacy of the EU will be developed. The proposal is that European public sphere research should focus on the more active role of the media as an independent variable that affects institutional choices and processes. Empirical results from comparative content analyses are discussed, which illustrate to what extent media have become an enabling and/or constraining factor on European integration.