Optical effects and photocatalytic behavior of semiconductor particles depend strongly on their size. The conduction and valence band of quantized semiconductor particles exhibits stronger reduction and oxidation potentials, respectively, compared to the bulk material. As a result of such changes, the charge - transfer processes on the surface of small semiconductor clusters are enhanced. The behavior of conduction band electrons (e_CB^-) and valence band holes (h_VB⁺) of metal - chalcogenide, metal-halogenide and oxide type nanometer - sized semiconductor particles are investigated using the techniques of pulse radiolysis and laser flash photolysis. The environmental conditions of the surface of colloidal particles influence the trapping of excess electrons and holes, and thus influence overall charge - transfer processes. Photocatalytic properties of quantized semiconductor particles and factors that effect the charge - transfer on the semiconductor/electrolyte interface are discussed here.