Sulfonamide antibiotics (SAs) are emerging pollutants that are ubiquitous in the aqueous environment. They are of acute concern as their pseudo-persistence and many of them can induce environmental bacterial resistance. The molecular structures of the antibiotics contain both ionizable groups and heteroatoms. Theoretically, they can undergo acid-base dissociation and exist in different dissociation forms in the aqueous environment. Photochemical degradation has been proved to be a significant elimination pathway of SAs in sunlit natural waters. However, the environmental behavior of the antibiotics in different dissociation forms is still unclear, especially for their photooxidation by the reactive oxygen species (ROS). Therefore, in this study, sulfamethazine (SMZ) was adopted as a case to study the photooxidation kinetics of individual dissociation forms with hydroxyl radicals (·OH) and singlet oxygens ( 1 O 2 ), and to investigate the corresponding intermediates and environmental fate. Competition kinetic experiments and matrix calculations were performed to determine the second order reaction rate constants of the dissociation forms with ·OH and 1 O 2 . The results indicated that their bimolecular rate constants for the reaction between SMZ and ROS increased with the increasing matrix pH. The three SMZ dissociation forms showed different ROS photooxidation activities, which was enhanced from cationic states to anionic states. The reaction activities in different dissociation forms with 1 O 2 varied by orders of magnitude, and the anionic states were most easily to be oxidized by 1 O 2 . In the normal pH range of natural waters (6.0−9.0), the corresponding environmental half-lives of SMZ were calculated to be 21.17−32.12 h for ·OH oxidation and 0.06−1.06 h for 1 O 2 oxidation, respectively. Compared with direct photolysis and ·OH oxidation, 1 O 2 oxidation may be a central factor in determining the environmental photochemical fate in sunlit surface waters. Furthermore, the main photooxidation products were enriched by solid phase extraction and identified by Agilent 6410B triple quadrupole liquid chromatography-mass spectrometry. The different intermediates and pathways were involved for ·OH oxidation, 1 O 2 oxidation, and direct photolysis. One single product was formed from 1 O 2 oxidation, while diverse hydroxylated products were generated from ·OH oxidation. The present study found that SMZ showed new photochemical behavior: multiple ROS oxidation kinetics, pH dependent environmental half-lives, and different intermediates for the three photochemical reactions. These findings are of importance toward the goal of understanding the multivariate photochemical behavior of different antibiotic dissociation forms in surface waters. Thus, to assess the environmental fate and ecological risk of organic pollutants like SAs, the effects of acid-base dissociation must be considered.