Photocatalysis is one of the advanced oxidation techniques that are being studied for the treatment of polluted air and water from different sources. From a kinetic point of view, photocatalytic reaction rates are strongly dependent not only on the reactant and product concentrations, but also on the rate of photon absorption. Unfortunately, the local rate of photon absorption is usually difficult to evaluate because of (i) the inherent complexity of the system and (ii) the lack of data concerning the photocatalyst optical properties. The final objective of this project is focused on the development of a complete model of the radiation field; the bed structure, and the flow pattern to describe the operation of a fixed bed photocatalytic reactor. In this article, the interaction between radiative energy and TiO2-coated fused-silica sphere beds was studied. The proposed model was built applying the Monte Carlo method, taking into account the complex reflection/refraction/absorption interactions between radiation and the packed bed. To obtain experimental measurements, an ad hoc device was designed and built. This device allows us to validate the proposed radiation model, and to obtain the optical parameters of the composite photocatalyst, i.e., the refractive index and the surface rough index of the fused-silica spheres, as well as the refractive index and the optical thickness of the TiO2 films. (c) 2007 American Institute of Chemical Engineers.