Abstract: The previously not studied photochemical degradation of sulfamethoxazole (SMX) to the isomer of SMX (ISO) was measured via a polychromatic (Xe) and a monochromatic (Hg) light source and accompanied by quantum chemical DFT calculations. In addition to the pKa=7.0±0.1 of ISO, tautomer-dependent properties such as the K_OW were measured and theoretically confirmed by DFT. The kinetics in solutions below and above the p K_a= 5.6 of SMX were studied for the available and quantifiable products SMX, ISO, 3-amino-5-methylisoxazole (AMI), 2-amino-5-methyloxazole (AMO), and sulfanilic acid (SUA). The quantum yields of the neutral (Φ _N) and anionic Φ _A) forms of SMX (Φ _A= 0.03 ± 0.001 , Φ _N= 0.15 ± 0.01) and ISO (Φ _A= 0.05 ± 0.01 and Φ _N= 0.06 ± 0.02) were found to be wavelength-independent. In a competitive reaction to the formation of ISO from SMX, the degradation product TP271 is formed. Various proposed structures for TP271 described in the literature have been studied quantum mechanically and can be excluded for thermodynamic reasons. In real samples in a northern German surface water in summer 2021 mean concentrations of SMX were found in the range of 120 ng/L. In agreement with the pH-dependent yields, concentrations of ISO were low in the range of 8 ng/L. Graphical abstract: [Figure not available: see fulltext.]