In photochemistry, an accurate description of the radiation field is important for successful modeling and subsequent design of a photocatalytic reactor, since the radiation field determines the local volumetric rate of energy absorption, which in turn controls the photocatalytic reaction rate. However, scattering by catalyst particles present in water causes the photon energy to be redistributed and the light field in a heterogeneous photoreactor is consequently difficult to evaluate. Light field in a heterogeneous photoreactor is determined by selected optical properties of the catalyst particle, such as scattering and absorption coefficients. However, so far there has been no direct experimental method to measure these two values. In addition, these two coefficients alone are not sufficient to describe the effect of catalyst on the light field in a catalyst suspension. A third optical property of scattering mode, which gives the spatial distribution of the scattering directions, has been found to be important in parallel light field. In this study, a new light model along with the optical properties of two commercially available titanium dioxide (TiO2) catalysts was used to determine the light field in a test annular photoreactor for different concentrations of suspended TiO2. Chemical actinometry was used to measure the light intensity inside the reactor. The scattering mode of the catalysts was determined by fitting experimental data with the model predictions. It was found that the optical properties of the catalyst change significantly at different catalyst concentrations. This suggests that the application of universal optical properties of a catalyst may not be suitable for the determination of light field for a large range of catalyst concentrations. (c) 2005 Elsevier Ltd. All rights reserved.