This research augmented the detailed Activated Sludge Model (ASM) to include abiotic hybrid nitrous oxide (N2O) production due to reactions between hydroxylamine (NH2OH) and nitrite. Model parameters were extracted from a series of abiotic and biological batch tests that showed that abiotic NO production rates are 1-3 orders of magnitude greater than biological N2O production rates under observed experimental conditions. The mass transfer coefficients (kLa) were 19 h(-1) and 21 h(-1) for abiotic and biological tests, respectively. Second order reaction rate constants for N2O indicated that the presence of activated sludge enhanced the rate of abiotic hybrid N2O production. Experimental findings were supported by standard free energy estimates showing that the hybrid reaction (Delta G degrees hybrid = 538 kJ/mop is more thermodynamically favorable than hydroxylamine oxidoreductase (HAO)-mediated oxidation of NH2OH (i.e. Delta G degrees 247 kJ/mol). The revised model successfully predicted the ammonia-N removal rate (49 mg N/L-h) as well as the temporal position of the N2O production observed from independently conducted dynamic loading experiments. Incorporation of abiotic hybrid N2O production should significantly improve N2O emissions estimates from partial nitrification systems but it will also alter the material and energy balance for nitrifiers because these abiotic reactions effectively re-route NH2OH. (C) 2015 Elsevier B.V. All rights reserved.