This article reviews papers devoted to the investigation of photoinduced processes on non-semiconductor oxide catalysts. The main results of our ESR in situ studies of such systems are presented. Wide-band-gap oxides are shown to constitute a new and very promising class of photocatalysts capable of functioning under illumination with visual light. It is shown that in many cases the light is initially absorbed by charge transfer complexes with the following generation of ion-radical intermediates with high reactivity. A very important feature of photoinduced reactions in the absorption band of charge transfer complexes is a significant shift of their red edge to longer wavelengths in comparison with similar processes in homogeneous systems. In many cases, the formation of such complexes is caused by the existence of strong acceptor or donor sites typical for many oxide catalysts with pronounced acidic or basic properties. (C) 2000 Elsevier Science B.V. All rights reserved.