Optical properties of polymer-stabilized aqueous colloids of silver sulfide were studied. Nature and energies of optical transitions responsible for the absorption of visible light by silver sulfide nanoparticles were determined. Catalytic properties of Ag2S nanoparticles in methylviologen reduction by sodium sulfide were examined. It was shown that this reaction is reversible, the equilibrium caused by a reverse reaction between the product of direct catalytic process - sulfur (polysulfide anions) and cation-radical of methylviologen. It was found that Ag2S nanoparticles can catalyze reduction of Ag+ ions by various reductants, in particular, hydroquinone or sodium sulfite, with the formation of silver particles having characteristic plasmon absorption bands in the visible spectral domain. Effect of the conditions of Ag+ catalytic reduction on the shape and intensity of plasmon resonance bands of metallic silver were studied, schematic mechanism of this reaction was proposed. It was shown that at the irradiation of solutions containing Ag2S nanoparticles, Ag+ cations and a reducing agent with the light corresponding to the fundamental absorption band of nanoparticulate silver sulfide, the rate of Ag+ catalytic reduction increased whereas kinetic features of catalytic reaction and characteristics of plasmon resonance absorption bands of silver nanoparticles changed. A mechanism of photochemical activation of the catalytic reduction of silver ions with the participation of Ag2S nanoparticles was proposed. (C) 2004 Elsevier B.V. All rights reserved.