A proposal for scaling-up photocatalytic reactors is described and applied to catalytic walls coated with a thin layer of titanium dioxide and irradiated with near UV radiation. The method is exclusively based on the fundamentals of chemical reaction engineering and radiative transfer theory, without the use of adjustable parameters in going from the laboratory information to a pilot scale apparatus. Mathematical modeling has been utilized. From kinetic information obtained in experiments performed in a flat plate of 81 cm(2), operating at steady state in a continuous, well-mixed reactor with recycle, predictions for a continuous flow, multi-annular reactor having a catalytic surface of 5209cm(2) agree very well with the validation tests. Thus, the achieved scale-up implies a change in size, shape, configuration and operating conditions of both employed reactors. Requirements to apply satisfactorily the proposed methodology are reported in detail. The root mean square error in the verification of conversion predictions for the larger scale photocatalytic reactor when compared with experimental data is less than 5.6%. (c) 2006 Elsevier Ltd. All rights reserved.