The presence of nitrite during denitrification is generally related to N2O emissions. The aim of this work was to determine the operational conditions that have influences in the specific denitrifying activities (SDA) and N2O production. Two factorial experimental designs (2(3) and 2(2)) were performed to evaluate the effects of four operational variables (pH; biomass concentration; nitrite concentration and C/N ratio) on the SDA. Batch experiments were carried out with two different carbon sources: acetate and swine wastewater. Results showed that both biomass and NO2- concentrations had a significant effect on the SDA in the presence of acetate, in the case of swine slurry, pH also affected the SDA. N2O production was only detected when swine wastewater was used and its specific production rate increased with the increase of the SDA. This result would indicate that N2O emissions are also promoted when swine wastewater is treated in conditions that favor the SDA. (c) 2012 Elsevier Ltd. All rights reserved.