A fast kinetics study by pulsed laser spectroscopy allows a mechanism for Orange II photobleaching in the presence of O₂ to be proposed. Evidence is presented that the hydroperoxy radical HO₂ is the main oxidative species for Orange II in oxygenated aqueous solution under visible light irradiation. The probability of reaction between O₂ and Orange II leading to radical formation (≈5×10^-4) is observed to be very low. The lifetime and amplitude of the radicals anions and cations in solution are compared in O₂-, air- and Ar-containing solutions. Singlet oxygen (¹O₂) is shown not to be present during the initial photobleaching stages of Orange II. Steady-state irradiation of Orange II in aqueous solution in the presence of O₂ causes two simultaneous processes: photodegradation to more biodegradable intermediates with CO₂ evolution and photoinduced generation of peroxides in solution. The observed peroxide generation stops when initial Orange II reaches a low concentration and does not occur from the intermediates during the photodegradation of Orange II. Direct electron transfer from excited Orange II to O₂ provides the driving force for the accelerated photodegradation of the dye. No electron transfer to O₂ was observed in the dark involving the ground state of the azo dye. The oxidation potential Orange II/Orange⁺ is determined by cyclic voltammetry to be 0.76 V(NHE) and a value of -1.54 V(NHE) could be estimated for the oxidation potential of the couple Orange II*/Orange⁺.