The enlargement of he European Union was expected to increase the effect of national inequalities within the supranational decisional system and significantly difficult the resolution of problems of policy coordination. However, the decision-making process of the EU has shown a remarkable flexibility and capacity of adaptation in the face of the entrance of new members. In this paper, I theorize how coalition formation in the Council of Ministers contributes to this flexibility in the issue area of environmental policy. Using an extension of the Banzhaf Power Index, I explain how coalition formation incorporates the preference variations infused by new member states through the combination of re-weighted voting power and a strategic process of bilateral exchanges between coalitions and selectively targeted member states. The analysis is completed with a procedural model, which takes into account the influence of the EP in the legislative process. Without imposing unanimity, the configuration of a decisive coalition under QMV requires an attention to particularized interests. In contrast with propositions based on axiomatic models of bargaining, I argue that the focus on the power-based coalition formation can better explain outcomes of effective differentiated adaptation to the Eastern enlargement.