A new method for disperse dye decolourization using the photochemically active silicadodecatungstic acid (H4SiW12O40) + isopropanol (electron donor) redox system has been described. Results of a series of control experiments have indicated that the one-electron reduced form of the silicadodecatungstic acid, SiW12O405-, was responsible for reductive cleavage of disperse dye as evidenced by photochemical heteropoly blue generation upon H4SiW12O40 + isopropanol UV-illumination. A threshold (0.09 mM) and an optimum (0.5 mM) heteropoly acid dose existed for minimum and maximum decolourization rates, respectively. It was also evident that decolourization kinetics were first-order with respect to dye concentration for concentrations up to 75 mg/l, and the photochemical degradation rate became UV-light penetration limited (zero order) above 100 mg/l dye concentration. (C) 2003 Elsevier Ltd. All rights reserved.