The direct photolysis of 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) in aqueous solution was investigated under monochromatic ultraviolet (UV) irradiation at 254nm. ABTS was found to be directly photolyzed by UV irradiation to yield the one-electron oxidized radical, ABTS⁺, which is a blue-green colored persistent radical species that has strong visible absorption bands. The photochemical production of ABTS⁺ was strongly dependent on the solution pH and the presence of dissolved oxygen. The presence of dissolved oxygen increased the quantum yields at pH 3, whereas it inhibited the production of ABTS⁺ at pH 6.5. Spectrophotometric and spectrofluorometric data indicated that ABTS photolysis may occur as a result of the transfer of one-electron between the singlet excited state and the ground state of ABTS. Observations made during UV/H₂O₂ experiments with ABTS suggested that the dependence of the photoloysis of ABTS on the solution pH and the presence of dissolved oxygen is related to the role of the hydroperoxyl/superoxide radical (HO₂/O₂⁻), which appears to be formed via a secondary reaction of the reduced intermediate of ABTS with dissolved oxygen. The proposed photolytic reactions were supported by the observed stoichiometry between the amount of ABTS+ radicals produced and the amount of ABTS molecules decomposed.