To treat dyeing and finishing industrial wastewater effluents by an advanced oxidation process, the decomposition kinetics of methylene blue by nanofine sol TiO2/UV was systematically studied in two batch slurry reactors. The factors of study include agitation speed, recirculation flow rate, initial dissolved oxygen concentration, initial methylene blue concentration, reaction temperature, TiO2 dosage, and UV light intensity. By using the network reduction technique, the general rate equation has been developed from considering the reaction mechanism. The rate equation uses a systematic reaction network reduction technique to avoid the need of more restrictive assumptions commonly used, such as a rate-determining step or equilibrium steps. The resulting rate equation is applicable to a wider range of reaction conditions. After applying the resulting rate equation to predict the transient concentrations of other dye compounds, we expect it can be applied for the kinetics of photodecomposition of other organic compounds.