The use of stereolithography is well suited for the fabrication of a monolithic photocatalyst for cleaning of air contaminated by volatile organic compounds (VOCs). In this paper we present the influence of the geometry of the monolith on the reactor's efficiency. Our aim is to develop a model that describes the basic phenomena, which are involved, i.e., light absorption, hydrodynamic and transfer processes and the reaction kinetics. Three different geometries of the monolith reactor have been tested: mixer (M), crossed channels (C), and star geometry (S). It appears that the geometry has practically no influence on the external mass transfer rate but has a great influence on the kinetics of photocatalysis. The model will be used to predict and optimize the photocatalytic behaviour and to scale up the results to an industrial reactor. (c) 2007 Elsevier Ltd. All rights reserved.