Laboratory-scale experiments were conducted to examine the N2O emission during the denitrification process. For each of the 6 runs carried out, synthetic effluent was fed in a 10 1 batch mixed liquor to investigate the effect of nitrite on N2O emission and Helium was continuously bubbled through the reactor at constant rate (0.12 l/min) to favour N2O transfer and detection. An increasing COD/NO3--N influent ratio from 3 to 7 was firstly applied (runs 1-3). Secondly, NO2- Pulse additions were performed during run 4 and 5 (10 and 20 mg N/l, respectively). Finally, the reactor was fed with influent containing both NO2- and NO3-. We showed that N2O emission was detected shortly after NO2- accumulation, few minutes after the substrate feeding. The highest emission occurred at the lower COD/NO3--N ratio (=3) and at the higher NO2- addition (20 mg N/l). In addition, the higher nitrogen conversion to N2O gas (14.4%) was obtained with an influent containing initially both NO2- and NO3-. Our results suggest a direct effect of the NO2- concentration on the N2O emission. We have also confirmed the inhibitory effect of NO2- concentration on N2O reduction. (C) 2008 Elsevier Ltd. All rights reserved.