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 (NO₃⁻) and ammonium (NH₄⁺). In batch ozonation experiments, the formation of NO₃⁻ and NH₄⁺ was investigated for natural organic matter standards, surface water, and wastewater effluent samples. A good correlation was found between NO₃⁻ formation and the O₃ exposure (R² > 0.82) during ozonation of both model DON solutions and real water samples. To determine the main precursor of NO₃⁻, solutions composed of tannic acid and model compounds with amine functional groups were ozonated. The NO₃⁻ yield during ozonation was significantly higher for glycine than for trimethylamine and dimethylamine. Experiments with glycine also showed that NO₃⁻ was formed via an intermediate with a second-order rate constant of 7.7 ± 0.1 M⁻¹s⁻¹ while NH4+ was formed by an electron-transfer mechanism with O₃ as confirmed from a hydroxyl radical (OH) yield of 24.7 ± 1.9%. The NH₄⁺ concentrations, however, were lower than the OH yield (0.03 mol NH₄⁺/mol OH) suggesting other OH-producing reactions that compete with NH₄⁺ formation. This study concludes that NO₃⁻ 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.