Dissolved organic nitrogen (DON) is an emerging concern in oxidative water treatment because it exerts oxidant demand and may form nitrogenous oxidation/disinfection by-products. In this study, we investigated the reactions of ozone with DON with a special emphasis on the formation of nitrate (NOT) and ammonium (NH4+). In batch ozonation experiments, the formation of NOT3- and NH4+ was investigated for natural organic matter standards, surface water, and wastewater effluent samples. A good correlation was found between NOT formation and the O-3 exposure (R-2 > 0.82) during ozonation of both model DON solutions and real water samples. To determine the main precursor of NON, solutions composed of tannic acid and model compounds with amine functional groups were ozonated. The NOT yield during, ozonation was significantly higher for glycine than for trimethylamine and dimethylamine. Experiments with glycine also showed that NOT was formed via an intermediate with a second-order rate constant of 7.7 +/- 0.1 M(-1)s(-1) while NH4+ was formed by an electron-transfer mechanism with O-3 as confirmed from a hydroxyl radical ((OH)-O-center dot) yield of 24.7 +/- 1.9%. The NH4+ concentrations, however, were lower than the OH yield (0.03 mol NH4+/mol center dot OH) suggesting other OH-producing reactions that compete with NH4+ formation. This study concludes that NOT formation during ozonation of DON is induced by an oxygen transfer to nitrogen forming hydroxylamine and oxime, while NH4+ formation is induced by electron transfer reactions involving C-centered radicals and imine intermediates. (C) 2016 Elsevier Ltd. All rights reserved.