Photophysical characteristics of colloidal CuS nanoparticles synthesized in various conditions and stabilized in aqueous solutions with sodium polyphosphate (SPP) were studied. A correlation between the band gap of CuS nanoparticles and their average diameter was established. Catalytic activity of colloidal CuS nanoparticles in sodium sulfide air oxidation in aqueous solutions at atmospheric pressure and room temperatures was established and thoroughly investigated. Kinetics of HS- oxidation and the nature of principal products of this reaction (thiosulfate and sulfate ions) were elucidated. A scheme for the mechanism of hydrosulfide ions catalytic oxidation was proposed. Accordingly to the scheme, HS- oxidation is a chain-radical reaction initiated on the surface of CuS nanoparticles and propagating further in the bulk of a solution. (C) 2003 Elsevier B.V. All rights reserved.