We calculate the distribution of absorbed radiation inside a solar photocatalytic reactor, by means of radiative transfer theory. The reactor configuration is that of a glass tube illuminated by a parabolic trough collector, where the catalyst consists of titanium dioxide micro-particles suspended in water. The calculations are made within the framework of the P1 approximation, which allows to solve analytically the radiative transfer equations. The obtained Solution is Used to Study the effect of catalyst concentration on the degradation of pollutants, by means of a general kinetic model. The results obtained display the main features which are observed in experiments reported in the literature. (C) 2003 Elsevier Ltd. All rights reserved.