Colloidal semiconductor particles may act as efficient photocatalysts for a range of environmentally and industrially useful reactions. The primary step in these reactions is the absorption of ultra-band gap energy photons by the particles, which generates electron-hole ( ) pairs within the semiconductor lattice. The valence band holes/conduction band electrons may recombine or diffuse to the semiconductor surface where they may either reduce/oxidise particle lattice sites or undergo interfacial electron transfer with a surface adsorbed substrate or species in solution1. Photocatalytic treatment of metal ion species by such particles has a number of important commercial applications in precious metal recovery2 and in the removal of heavy elements from effluent streams3. The purpose of the work reported in this communication is to investigate the potential of heterogeneous photocatalysis as a mean of controlling the valence state of key actinide and metal species in such a way as to improve the performance of nuclear fuel reprocessing and decontamination technologies. For example, colloidal photocatalysis could be used to convert the valence state of actinides to insoluble species, which may then be selectively removed from solution. In this paper, we report some results obtained for the photoreduction of species such as cerium, a lanthanide whose thermodynamic Eh-pH diagram most closely approximates that of plutonium, and uranium as well as a kinetics analysis of the cerium system.