A laboratory-scale sequencing batch reactor fed with synthetic high-ammonium wastewater was operated for autotrophic partial nitritation, and the characteristics of N2O emission were investigated by both macro- and microanalysis. A N2O emission quantity of 5.98 +/- 1.24 mg (n = 17) was observed, accounting for 0.997% +/- 0.408% of the influent nitrogen. The N2O level in the off-gas first increased to a maximum of 91.5 ppmv in the initial 50 min, and then decreased and remained around 34.8 ppmv until the end of aeration. An asymptotic function (y = 143-14.2 x 0.992(x), R-2 = 0.997) could be used to estimate the quantity of N2O emitted. Microelectrodes were used to explore the variations in the nitrogen distribution and consumption rate within sludge flocs as the reaction proceeded. During the aeration phase, partial nitritation occurred mainly in the surface layer of flocs. This is where most N2O was produced, indicating that N2O was generated as a byproduct of the nitritation. During the settling phase, the N2O concentration and volumetric production rate were much higher than those in the aeration phase, suggesting that the settling phase contributes to the initial N2O emission peak. (C) 2015, The Society for Biotechnology, Japan. All rights reserved.