Abstract. It is argued that enlargement challenges institutional balances and in particular relative powers of national actors within the European Union (EU). This article concentrates on the impact of future enlargement (with the current negotiating 12 candidates) on power distribution in the Council of Ministers of the European Union and the European Parliament based on the decisions taken at the Nice Summit in December 1800. It uses the Shapley-Shubik and Banzhaf indices to evaluate past and emerging power distributions in both the Council and in the Parliament. A brief section on Turkey (the thirteenth, non-negotiating, official candidate) is included to evaluate its possible impact in the case of admission to the Union.