The environmental micropollutant sulfamethoxazole (SMX) is susceptible to phototransformation by sunlight and UV-C light which is used for water disinfection. Depending on the environmental pH conditions SMX may be present as neutral or anionic species. This study systematically investigates the phototransformation of these two relevant SMX species using four different irradiation scenarios, i.e., a low, medium, and high pressure Hg lamp and simulated sunlight. The observed phototransformation kinetics are complemented by data from compound-specific stable isotope and transformation product analysis using isotope-ratio and high-resolution mass spectrometry (HRMS). Observed phototransformation kinetics were faster for the neutral than for the anionic SMX species (from 3.4 (LP lamp) up to 6.6 (HP lamp) times). Furthermore, four phototransformation products (with m/z 189, 202, 242, and 260) were detected by HRMS that have not yet been described for direct photolysis of SMX. Isotopic fractionation occurred only if UV-B and UV-A wavelengths prevailed in the emitted irradiation and was most pronounced for the neutral species with simulated sunlight (εC = -4.8 ± 0.1 ‰). Phototransformation of SMX with UV-C light did not cause significant isotopic fractionation. Consequently, it was possible to differentiate sunlight and UV-C light induced phototransformation of SMX. Thus, CSIA might be implemented to trace back wastewater point sources or to assess natural attenuation of SMX by sunlight photolysis. In contrast to the wavelength range, pH-dependent speciation of SMX hardly impacted isotopic fractionation.