The photocatalytic degradation of metolachlor in water and butyric acid in air on coated TiO2 has been investigated to study the interdependency on the degradation rate of the UV photon flux and the concentration of the organic compound. Experimental results clearly showed that the kinetic change of order with respect to light intensity depended on the value of the concentration of organic compound. The fitting of experimental results with the Langmuir-Hinshel wood (L-H) model emphasized that the corresponding "apparent adsorption constant", K-R, could not be considered as an equilibrium constant in the dark since the value of KR varied with regard to the photon flow. The assumption of a pseudo-steady-state for the concentration of bydroxyl radicals associated either with L-H model (under certain conditions) or Eley-Rideal model explains consistently the dependence of the apparent kinetic parameter, k(obs), and K-R with the light intensity. A rate expression taking into account the interdependency of the degradation rate on the UV photon flux and the initial concentration of the organic compound is proposed and validated for a liquid and a gas phase reaction. The constants of this correlation are independent of the initial concentration and light intensity. (c) 2007 Elsevier B.V. All rights reserved.