Electrons and positive holes have been produced in colloidal TiO₂ particles upon illumination with near-u.v. light. In the absence of adsorbed reactants, the charge carriers recombine. In the presence of an adsorbed reactant for holes [such as SCN^– or poly(vinyl alcohol)], an excess of long-lived electrons remains on the particles and finally reacts with a reactant in the bulk solution. In the presence of an adsorbed reactant for electrons (such as a platinum deposit or methyl viologen), an excess of long-lived holes remains and finally reacts with an oxidizable compound. The processes observed include the reduction of water to form H₂, the reduction of tetranitromethane and halothane and the oxidation of Br^– and various organic compounds. Methyl viologen, MV²⁺, was found to be reduced to the radical MV^˙+ and also to be oxidized, to a lesser extent, to form a fluorescing compound. The absorption spectra of the long-lived excess of electrons and positive holes trapped at the interface were measured using flash photolysis.