In order to clarify the dependence of apparent adsorption constant Ks in the Langmuir-Hinshelwood (L-H) model on the incident light intensity, photocatalytic degradation kinetic characteristics were experimentally investigated at different light intensities using para-chlorobenzoate (4-CBA) as a model compound. In all cases 4-CBA initial degradation rates showed dependence on 4-CBA initial concentration, which can be described by the L-H model. However, the adsorption constant Ks and rate constant kr obviously varied with light intensity. To account for the experimental finding, slight modification of the classic kinetic model developed by Turchi and Ollis was tentatively proposed by assuming insufficiency of H2O/OH-available for photo-activated holes' scavenging. Such a kinetic model predicts that both kr and Ks-1 are linearly proportional to the square root of the intensity in a rather large intensity range. The validity of the modified model was proved by fitting it to the experimental data. #2002 Elsevier Science Ltd. All rights reserved.