There is increasing evidence showing that ammonia-oxidizing bacteria (AOB) are major contributors to N2O emissions from wastewater treatment plants (WWTPs). Although the fundamental metabolic pathways for N2O production by AOB are now coming to light, the mechanisms responsible for N2O production by AOB in WWTP are not fully understood. Mathematical modeling provides a means for testing hypotheses related to mechanisms and triggers for N2O emissions in WWTP, and can then also become a tool to support the development of mitigation strategies. This study examined the ability of four mathematical model structures to describe two distinct mechanisms of N2O production by AOB. The production mechanisms evaluated are (1) N2O as the final product of nitrifier denitrification with NO?2- as the terminal electron acceptor and (2) N2O as a byproduct of incomplete oxidation of hydroxylamine (NH2OH) to NO?2-. The four models were compared based on their ability to predict N2O dynamics observed in three mixed culture studies. Short-term batch experimental data were employed to examine model assumptions related to the effects of (1) NH?4+ concentration variations, (2) dissolved oxygen (DO) variations, (3) NO?2- accumulations and (4) NH2OH as an externally provided substrate. The modeling results demonstrate that all these models can generally describe the NH?4+, NO?2-, and NO?3- data. However, none of these models were able to reproduce all measured N2O data. The results suggest that both the denitrification and NH2OH pathways may be involved in N2O production and could be kinetically linked by a competition for intracellular reducing equivalents. A unified model capturing both mechanisms and their potential interactions needs to be developed with consideration of physiological complexity. Biotechnol. Bioeng. 2013; 110: 153163. (C) 2012 Wiley Periodicals, Inc.