The effect of oxygen on the reaction kinetics of the photocatalytic oxidation of dichloroacetic acid at pH below the titanium dioxide isoelectric point (pH from 5 to 4) was studied. A kinetic scheme based on the direct hole attack to the dichloracetate ion in conjunction with the classical role of oxygen acting as an electron acceptor is proposed. Oxygen also intervenes in a direct addition reaction to one of the intermediate reaction products. Experiments were conducted in a well-mixed, small, flat plate reactor employing Aldrich titanium dioxide as the photocatalyst in the suspension mode and using polychromatic irradiation with energy in the range from 275 to 385 nm in wavelength. A complete mathematical model, including the effect of the absorbed radiation intensities and catalyst concentration was developed. Experimental data agree well with theoretical predictions employing just two kinetic parameters derived from the proposed reaction mechanism. (c) 2005 Elsevier Ltd. All rights reserved.