The intfinsic reaction kinetics of the photocatalytic decomposition of dichloroacetic acid (DCA) at different, but constant oxygen concentrations was studied. Other significant variables were initial DCA concentration, catalyst (titanium dioxide) concentration and light intensities. The results of the kinetic measurements were analyzed using a complete mathematical model of the experimental device that was used for the laboratory operation (a differential reactor inside a recycle). In this way it was expected to obtain intrinsic kinetic parameters. The kinetic model, that was developed from a proposed complete reaction sequence, has only two lumped specific kinetic constants and represents the dependence of the reaction rate with oxygen, DCA and catalyst concentrations as well as the local volumetric rate of photon absorption. Simulation results are in good agreement with experiments and, at the same time, permit a detailed description of the existing radiation field in the experimental reactor. (c) 2005 Elsevier Ltd. All rights reserved.