Mathematical models for nitrous oxide (N2O) production by ammonia oxidizing bacteria (AOB) based on a single pathway have been proposed to support the design and operation of wastewater treatment plants (WWTPs). However, the boundary conditions for each of these models have not been established to date. This study tests the predictive ability of two single-pathway models based on the AOB denitrification pathway and the hydroxylamine (NH2OH) oxidation pathway, respectively, to describe the N2O data generated by a N2O model that incorporates both pathways, and provides theoretical guidance on how to use these two single-pathway models as well as the two-pathway model under various conditions. The model based on the AOB denitrification pathway can be used under the condition of a constant dissolved oxygen (DO) concentration, applied either at a low DO concentration (>=similar to 0.5 mg O-2/L) with any non-inhibitory nitrite (NO2-) concentrations or at higher DO (>=similar to 0.5 mg O-2/L) with relatively high NO2- (>=similar to 1.0 mg N/L) but non-inhibitory concentrations. The model based on the NH2OH oxidation pathway can be applied under the condition of relatively high DO concentrations (>=similar to 1.5 mg O-2/L), being either constant or time-varying, with any non-inhibitory NO2- concentrations. Under other conditions, the two-pathway model should be applied. (C) 2015 Elsevier B.V. All rights reserved.