AbstractOptical properties of extremely small metal chalcogenide particles that exhibit quantization effects have been examined. For extremely small semiconductor clusters with low effective carrier mass, optical absorption edges are shifted by several volts as a result of quantization effects that increase the effective bandgap of particles. Injection of electrons or holes into quantized semiconductor colloidal particles was studied by the pulse radiolysis technique. The reducing potential of photogenerated carriers was found to be greatly enhanced in quantized semiconductor particles, compared to the corresponding bulk material. Excess electrons trapped on the surface lead to a blue‐shift in the absorption edge of colloids. The appearance of this shift depends critically on the method of colloid preparation. The presence of OH−ions in PbS, CdS, and CdTe colloidal solutions blocks the sites where electrons are trapped and shortens the lifetime of bleaching, whereas in a slightly acidic solution long‐lived bleaching was observed. Different spectroscopic properties were found for hole excess and OH adducts of hydrous titanium oxides as a consequence of the existence of different environmental conditions of Ti(IV) atoms at the surface. The position where hole injection occurs lies in the mid‐gap level of 25