This policy brief investigates the decision-making impact of admitting Bulgaria, Romania, Turkey and Croatia into the EU-25, focusing on the EU’s ‘capacity to act’ and the power distribution among the member states. The enlargement is projected to have relatively little impact on the EU’s capacity to act, as long as the Constitutional Treaty (CT) voting rules come into effect, but if the CT is rejected, enlargement would cripple EU decision-making. Turkish membership is calculated to have a big impact on the power distribution among member states. Under the Nice or CT voting rules, Turkey would be the second-most powerful member state in an EU-29. Under the CT rules, Turkey would be substantially more powerful than France, Italy and the UK, while under the Nice rules, the power differences among the member states with populations of more than 50 million would be small. Plainly, this distribution of power might decrease the acceptability of the Constitutional Treaty or Turkey’s membership (or both).