The recent enlargement of the European Union (EU) to 10 states, occurred on 1 May 2004, has been surrounded – both in academic and political circles – by two contrasting discourses. The first, prior to enlargement, foresaw dramatic consequences had the expansion not been accompanied by a serious, large-scale effort to reform its institutions. The second, subsequent to enlargement, tended on the contrary to downplay these predictions: despite the little ambition of the Treaty of Nice – so the argument goes – the entry of several new members has not altered the nature of the system, if not for some minimal logistic aspects. In fact, a serious, retrospective assessment of the EU after enlargement has not been performed by its own institutions; researchers who embarked on this exercise raise serious caveats about the significance of their data; political leaders complain that reform is badly needed in an enlarged EU and practitioners report a widespread tendency towards a more informal decision-making process. This study intends to contribute to this debate by developing standards and providing new evidence for a more comprehensive assessment. Results are surprising.