Excited triplet states of dissolved organic matter ((DOM)-D-3*) are important players for photodegradation sulfonamide antibiotics (SAs) in sunlit natural waters. However, the triplet-mediated reaction mechanism was poorly understood. In this study, we investigated the reaction adopting sulfadiazine as a representative SA and 4-carboxybenzophenone (CBBP)as a proxy of DOM. Results showed that the excited triplet state of CBBP ((CBBP)-C-3*) is responsible for the photodegradation of sulfadiazine. The reaction of (CBBP)-C-3* with substructure model compounds verified there are two reaction sites (amino-or sulfonyl-N atoms) of sulfadiazine. Density functional theory calculations were performed, which unveiled that electrons transfer from the N reaction sites to the carbonyl oxygen atom of (CBBP)-C-3* moiety, followed by proton transfers, leading to the formation of sulfadiazine radicals. Laser flash photolysis experiments were performed to confirm the mechanism. Thus, this study identified that the photodegradation mechanism of SAs initiated by (DOM)-D-3*, which is important for understanding the photochemical fate, predicting the photoproducts, and assessing the ecological risks of SAs in the aquatic environment. (C) 2015 Elsevier B.V. All rights reserved.