This study presents a kinetic model for the photocatalytic degradation of an organic pollutant in a fixed-film reactor for water treatment. The pharmaceutical drug clofibric acid (CA) was chosen as the model compound. Experiments were carried out employing TiO2 Aeroxide P25 immobilized on the glass window of a laboratory scale reactor under UV radiation. A kinetic model to represent the degradation of CA was proposed, which explicitly takes into account the absorption of radiation by the catalytic film. Experimental results for different irradiation conditions and different number of TiO2 coatings were used to estimate intrinsic kinetic parameters. External and internal mass transfer limitations were found to be negligible under our operating conditions. Good agreement was obtained between experimental data and model predictions, with a root mean square error of 9.9%. (C) 2015 Elsevier B.V. All rights reserved.