Starting in 2005, the EU will implement a CO2 emissions trading scheme. We show that the outspoken objectives of economic efficiency and free allocation of allowances are incompatible with harmonized allocation rules. The latter would be necessary to avoid unequal changes of the financial positions between identical firms across the EU, thereby distorting competition, i.e. the “level playing field”. We discuss and evaluate potential adjustments to the current prescriptions of the EU emissions trading system in order to achieve harmonization of allowance allocation across EU Member States. The proposed adjustments involve relaxation of either the efficiency objective or the objective of free allowance allocation.