The decomposition of sodium anthracene-1-sulfonate (AN1S) by irradiation (lambda > 300 nm) was investigated in oxygen-saturated aqueous solutions and TiO2 suspensions. The concentration decrease of AN1S during exposure follows the formal first-order kinetics. The formation of phthalic and salicylic acids was monitored by high performance liquid chromatography (HPLC) in both photosystems. Additionally, the formation of alizarin during irradiation in the presence of TiO2 was confirmed. The effect of experimental conditions (initial AN1S concentration, pH value, TiO2 concentration) on the formal initial rate of AN1S photodegradation was studied in details. In situ EPR spin trapping experiments in TiO2 suspensions confirmed the competitive reaction of photogenerated hydroxyl radicals between 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trap and AN1S.