This article reviews emergent literature on the effects of Eastern enlargement on EU decision-making. After recalling pre-enlargement hypotheses we inspect empirical findings on that issue. Whereas some authors stress a 'business as usual' reading of post-enlargement decision-making, others point out changes. In order to better understand the processes of post-enlargement governance we draw on three sociological group theories -oligarchization, formalization and adaptationand assess their applicability to the case of the EU. While we present only preliminary evidence for each of these theories, our secondary literature analysis underlines that the complexity of enlargement demands a widening of the theoretical scope. In such a perspective enlargement research can contribute to more general debates about decision-making in the EU.