Two kinds of polymer-stabilized metal nanocluster catalysts were prepared by different methods. One is homogeneous poly(acrylic acid)-stabilized silver (PAA-Ag) nanoclusters, which were prepared upon UV irradiation of an alcohol/water solution of silver(I) perchlorate in the presence of poly(acrylic acid). Oxidation of ethylene, catalyzed by PAA-Ag nanoclusters, was performed in glycol under I atm of ethylene/oxygen (2/1 v/v). Addition of both cesium(I) and rhenium(VII) ions remarkably increased the catalytic activity of PAA-Ag nanoclusters. The other is heterogeneous palladium nanoclusters, which were immobilized on chelate resin-metal ion complexes, and prepared by reduction of palladium(II) ions supported on resin complexes. In this heterogeneous catalyst, the chelate resin works both as a support and as a protective polymer of nanoclusters. Hydrogenation of olefins in ethanol under I atm of hydrogen at 30 degreesC was used for evaluating the catalysis of immobilized Pd nanoclusters. An increase of the reaction rate by lanthanoid ions was observed in the hydrogenation of olefins catalyzed by Pd nanoclusters supported on resin complexes.