The radiation field for an annular-type photocatalytic reactor has been modeled, and the resulting integro-differential equation has been solved resorting to the discrete ordinate method. The spatial domain was described in terms of two cylindrical coordinates and the field of direction with two angular variables. The radiation entering through the inner reactor wall was obtained from the extended source with voluminal emission model. The system parameters for the radiative-transfer equation were taken from experimental measurements. Resorting to computational experiments, the effects produced by different concentrations of (1) transparent scattering centers and (2) radiation absorbing (catalytic) particles were investigated. In the second case, Aldrich and Degussa P 25 titanium dioxide particles were used. The following information is reported : (1) specific intensities as a function of position inside the reactor and of the angular distribution of directions; (2) incident radiation profiles and local volumetric rate of energy absorption profiles, both as a function of the spatial position inside the reactor.