Nitrous oxide (N2O) emissions from biological denitrification processes are one of the major sources of N2O emissions during the biological nutrient removal process in wastewater treatment. In this study, the effects of carbon source shocks on N2O production during denitrifying phosphorus removal were investigated using biomass that was initially acclimated with acetate. The shock tests were conducted in three laboratory-scale anaerobic/anoxic (An/A) reactors using acetate, acetate/propionate or propionate as the carbon sources. After switching the carbon source from acetate to acetate/propionate and propionate, the ratios of N2O-N production to total nitrogen (TN) removal increased by 1.72 or 0.77 times, respectively, and the total nitrogen (TN) removal efficiency decreased from 80% to 52% or 38%, respectively; also, the PO43--P removal efficiency declined from 75% to 63% or 47%, respectively. (C) 2011 Elsevier B.V. All rights reserved.