Nitrous oxide (N2O) is a significant greenhouse gas, and biological nitrogen removal systems have been shown to be a significant N2O source. To evaluate the control parameters for N2O emission in the wastewater treatment process, N2O emissions were compared in the activated sludge from anoxic-aerobic sequencing batch reactors (A/O SBRs) acclimated under different aeration rates, and fed with synthetic wastewater. Results showed that a higher aeration rate led to a smaller N2O emission, while reactors acclimated under mild aeration performed the best in terms of nitrogen removal efficiency. Most of the N2O was produced during the aerobic phase, regardless of the aeration rate. Trace studies showed that incomplete denitrification appeared to be the major process responsible for high N2O emission at a low aeration rate (Run 1), while incomplete nitrification was the reason for N2O emission at a higher aeration rate (Run 2 and Run 3). For enhancing the efficiency of nitrogen removal while lowering energy consumption and reducing N2O emission, the optimal aeration rate would be 2.7 L-air/(L-reactor . h), in terms of the synthetic wastewater used. (C) 2009, The Society for Biotechnology, Japan. All rights reserved.