In this paper we will try to demonstrate that in nanostructured materials the scaling of many of the material properties that are important for catalytic activity diverges as one approaches nanosize dimensions. We will discuss size-induced critical properties that include transport, polarizability and adsorption properties. The macroscopic context will be demonstrated in terms of three main applications of nanocrystalline, semiconducting, photocatalytic materials with particular emphasis on nanocrystalline TiO2. These applications include: liquid-junction photovoltaic solar cells; photodecomposition of organic compounds with nanocrystalline photocatalysts and light-induced wetting properties. (C) 2000 Elsevier Science B.V. All rights reserved.