A lab-scale sequencing batch reactor fed with real municipal wastewater was used to study nitrous oxide (N2O) emissions from simulated wastewater treatment processes. The experiments were performed under four different controlled conditions as follows: (1) fully aerobic, (2) anoxic-aerobic with high dissolved oxygen (DO) concentration, (3) anoxic-aerobic with low DO concentration, and 4) intermittent aeration. The results indicated that N2O production can occur from both incomplete nitrification and incomplete denitrification. N2O production from denitrification was observed in both aerobic and anoxic phases. However, N2O production from aerobic conditions occurred only when both low DO concentrations and high nitrite concentration existed simultaneously. The magnitude of N2O produced via anoxic denitrification was lower than via oxic denitrification and required the presence of nitrite. Changes in DO, ammonium, and nitrite concentrations influenced the magnitude of N2O production through denitrification. The results also suggested that N2O can be produced from incomplete denitrification and then released to the atmosphere during aeration phase due to air stripping. Therefore, biological nitrogen removal systems should be optimized to promote complete nitrification and denitrification to minimize N2O emissions. Biotechnol. Bioeng. 2011; 108: 2036-2045. (C) 2011 Wiley Periodicals, Inc.