The photo-oxidation of sodium 9,10-anthraquinone-2-sulphonate in aqueous and aqueous-alcoholic solutions has been studied by flash, continuous photolysis, and electron spin resonance (e.s.r.) techniques. Whereas in aqueous-alcoholic solutions it is well established that the photo-excited quinone abstracts a hydrogen atom from the alcohol by A*+ RCH2OH → AH·+ RĊHOH it is shown here that in aqueous solution the quinone reacts by A*+ A → A·++ A·– From the flash photolysis and e.s.r. observations a mechanism is proposed for the reaction in neutral aqueous solution which satisfactorily accounts for the products and the kinetics of the reaction as observed by oxygen absorption and by spectral measurements under continuous photolysis conditions.