Nitrous oxide (N2O) emission has been observed during denitrification of nitrate via nitrite as intermediate. With a laboratory-scale reactor (2.4 L), the N2O emission was characterized under a gradient of DO concentration from 0 to 0.7 mg/L, different ratio of nitrite versus nitrate and different nitrite feed mode. The N2O emission was influenced by the level of dissolved oxygen (DO) and nitrite accumulation. The higher DO level and the higher ratio of nitrite versus nitrate resulted in the higher N2O emission. Using nitrite as sole electron acceptor at the same loading rate, the sequence of N2O emission with three different feed modes was: pulse > step-wise > continuous feed. The N2O emitted in pulse feed reactors was 3.1-4.2 and 8.2-11.7 folds of that in the step-wise feed and continuous feed reactors, respectively. With continuous feed mode, the impact of DO concentration on the mass of N2O emitted was limited while the higher N2O emission occurred at the higher nitrite loading rate. (C) 2012 Elsevier B.V. All rights reserved.