This work describes the use of solar radiation to destroy an organic pollutant in aqueous solution, using the photo-Fenton reaction. Formic acid was chosen as the model substrate. A kinetic model that accounts for the dark (Fenton) and the light activated (photo-Fenton) reaction rates in a single mathematical expression has been used. Furthermore, to take into account the irradiated and the nonirradiated liquid volumes, a rigorous mass balance for each one of the reactant species has been applied. Under the adopted operating conditions, an organic pollutant conversion up to 81% was achieved after 1 h of operation. Predictions of the theoretical model were compared with experimental results and good agreement for formic acid and hydrogen peroxide concentration evolutions was obtained. Deviations between model predictions and experimental data were always smaller than 9%. Theoretical and experimental results of the pollutant conversion showed that solar irradiation improves the effectiveness of the Fenton system significantly; a pollutant conversion 175% greater than that obtained with the dark system alone was observed. (C) 2004 Elsevier Ltd. All rights reserved.