The potential of a common semiconductor, ZnO, has been explored as an effective catalyst for the photodegradation of C.I. Acid Yellow 23 (AY23). The effects of process parameters such as, catalyst. loading, initial dye concentration, light intensity, and pH on the extent of photodegradation have been investigated. Substantial reduction of COD, besides removal of color, was also achieved. A rate equation for the degradation based on Langmuir-Hinshelwood (L-H) model has been proposed. The results show that the adsorption constant (K-ads) and rate constant (k(L-H)) in L-H model are dependent to the light intensity, and increase with increasing the light intensity. With inserting the light intensity parameter to L-H equation, this model can be used for predicting the removal rate at different light intensities and initial concentrations of AY23. A comparison between experimental and calculated apparent reaction rate constants shows that the results obtained from the L-H modified model are in good agreement with experimental data. (c) 2005 Elsevier B.V. All rights reserved.