Abstract: Phototransformation, a process from dissolved organic nitrogen (DON) to dissolved inorganic nitrogen (DIN), is an important component of nitrogen cycle in surface aquatic system. In this study, we measured photoammonification rates of typical low-molecular-weight DON (e.g., amino acids, nucleotides, and urea) under different conditions to elucidate the potential role of DON structures, light sources and water matrices during photoammonification. Results showed that under simulated sunlight, amino acids with aromatic moieties produced differentamounts of NH₄⁺. L-tyrosine and L-tryptophan exhibited the highest ammonification rates, up to 50% transformation within 6 hours, whereas there was negligible photoammonification rate of urea, adenine and cytosine. Under natural sunlight, L-tyrosine and L-tryptophan produced NH₄⁺ by phototransformation in both buffer solution and natural water sample, while all aromatic amino acids and L-histidine produced NH₄⁺ only in natural water sample by phototransformation. Visible light did not contribute to photoammonification of the low molecular weight DON. These results indicated that photoammonification was affected by light sources and matrix constituents. Meanwhile, the structures of DON affected photoammonification. For example, at optimal conditions of irradiation, all nitrogen atoms of L-histidine could be converted into NH₄⁺ by photoammonification, whereas only one of N atoms in L-tryptophan was transformed into NH₄⁺. Based on density functional theory calculation, there was significantly negative correlation between photoammonification rates and E_LUMO-E_HOMO of amino acids. Our results demonstrated the complex of DON photoammonification in surface water system. The significant role of photoammonification of low-molecular-weight DON in surface water system should be considered in nitrogen cycle, water quality evolution and the assessment of eco-environment.