Optical properties of solid catalysts needed for a rigorous modeling of heterogeneous photo-Fenton reaction kinetics and the ultimate reactor design have not been studied so far. Without these data, quantitative information concerning the influence of the radiation field in the process performance is almost impossible. These reactions are still under the first stages of their studies, and most of the effort has been aimed at elucidate yields, tentative mechanisms, the optimum operating pH and ways to reduce iron ions leaching to the aqueous solution. This work reports the values of above-mentioned parameters precisely calculated, resorting to specially designed spectrophotometer measurements coupled with the solution of the radiative transfer equation (RTE). Goethite, a natural occurring ferric oxide, was studied to obtain the values of the radiation linear volumetric absorption and scattering coefficients, as well as the phase function for scattering, some of them as a function of mass composition, and all of them as a function of the wavelength range between 3 10 and 500 nm. The results indicate that, in addition to its demonstrated ability to degrade effectively many contaminants, its optical properties exhibit very low absorption and scattering properties. This outcome improves its potentiality for applications in large-scale reactors, because the resulting useful optical path length will be significantly large. Specifically, these results are compared with a typical photocatalytic substance, such as titanium dioxide to remark the differences. (c) 2007 Elsevier B.V. All rights reserved.