The kinetics of degradation of Acid Red 3B photocatalyzed by PW12O(40)3- was studied in aqueous solution. Light intensity, dissolved oxygen, added amount of the catalyst, and the initial concentration of substrate had positive effects on the photodegradation rate of Acid Red 3B. The results were explained by Langmuir-Hinshelwood model, implying there was precomplexation between catalyst and substrate. Results compared to UV/TiO2 method indicated that heteropoly acid is an efficient photocatalyst.