This paper presents a dimensionless analysis of steady-state, continuous flow, photocatalytic reactors using "two-flux" (i.e., scattered photons are purely back scattered) and "six-flux" (i.e., scattered photons follow the route of the six directions of the Cartesian coordinates) radiation absorption-scattering models. The models retain the essential elements of a rigorous approach, whilst providing simple solutions. The effect of scattering albedo, omega and "apparent" optical thickness, tau(app), over reactant conversion in a flow-through photocatalytic reactor was analysed using the above models for three ideal flow conditions: (1) falling film laminar flow, (2) plug flow and (3) slit flow. The model simulations show that the optimum value of tau(app) that maximizes the conversion is a complex function of fluid flow and reaction kinetics and should be found in the range from 1.8 to 4.4. When comparing the models to experimental results for the photocatalytic oxidation of the herbicide "isoproturon" in a recirculation batch reactor, significant errors in predicting reactant conversion were found when neglecting the full effect of the radiation scattering phenomena. The experimental results were enclosed by the Beer-Lambert absorption model and the two-flux radiation model; however, the six-flux model provides the most accurate, yet simple, analysis of the radiation field in a photocatalytic reactor. (c) 2007 Elsevier B.V. All rights reserved.