This review deals with the preparation of highly reactive titanium oxide photocatalysts and the clarification of the active sites as well as the detection of the reaction intermediates at the molecular level. Furthermore, we discuss the advancement of photofunctional systems and processes that can utilize visible and/or solar light. The photocatalytic reactivity of semiconducting TiO2 powder was found to be dramatically enhanced by the loading of small amounts of Pt, which work to enhance the charge separation of the electrons and holes generated by light irradiation. Highly dispersed titanium oxide species prepared within zeolite frameworks or silica matrices showed unique photocatalytic performance much higher than that of conventional semiconducting TiO2 photocatalysts. The potential for the effective utilization and conversion of solar energy makes research into the modification of the electronic properties of TiO2 photocatalysts by such methods as advanced metal ion implantation to produce photocatalysts which are able to absorb and operate efficiently even under visible light irradiation one of the most important fields in photocatalysis research. This modification process can be applied not only to semiconducting TiO2 photocatalysts but also to TiO2 thin film photocatalysts, as well as titanium oxide photocatalysts highly dispersed within zeolite frameworks. Significantly, a new alternative method for directly preparing such visible-light-responsive TiO2 thin film photocatalysts; has been successfully developed by applying a RF magnetron sputtering deposition method. (C) 2003 Elsevier Science (USA). All rights reserved.