Heterogeneous photocatalysts, in the form of particles immobilized in permeable membranes or in layers covering supports, are under development for production of ultrapure water for use in semiconductor fabrication and other applications. Concentration and light intensity gradients present within a photocatalyst layer will cause local reaction rates to vary within the layer, complicating determination of intrinsic kinetics. Analytical solutions are obtained here for arbitrary networks of two or more species that undergo first-order reactions in photocatalyst layers. First-order or pseudo-first-order behavior should be obtained at the very low contaminant concentrations encountered in water ultrapurification. Experimental data can be analyzed using the solutions to obtain estimates of intrinsic rate coefficients for two flow configurations: (1) water and reactants flow through a photocatalyst layer, (2) water flows past one face of a photocatalyst layer into which reactants diffuse, with the other face sealed. A set of experimental data in the literature is analyzed and an estimate of the intrinsic reaction rate coefficient is obtained. The analytical solutions show that, when reactants flow through a photocatalyst layer, the same outlet composition is obtained for the same total incident light intensity, regardless of how the incident intensity is distributed between the two faces of the layer. When reactants diffuse into a photocatalyst layer, greater conversion can be obtained for light incident on the face over which water flows than for light incident on the sealed face. Greater conversion is obtained for reactant flow through a photocatalyst layer than for reactant diffusion into the layer. (C) 2003 Elsevier B.V. All rights reserved.