The electron-transfer reaction between hexacyanoferrate(III) ions and thiosulfate ions is known to be catalyzed by platinum nanoparticles. In the present study, the stability and catalytic activity of the PVP-Pt nanoparticle during its catalytic function for this electron-transfer reaction is studied. The stability of the nanoparticles after various perturbations was assessed using TEM, and the kinetics of the reaction was followed using absorption spectroscopy. The studies were conducted on four different concentrations of PVP-Pt nanoparticles. It was found that the average size and width of the PVP-Pt nanoparticles decrease slightly after the first and second cycles of the electron-transfer reaction. The size and size distribution width do not change in the presence of only the thiosulfate reactant, whereas the presence of only the hexacyanoferrate reactant results in a reduction of the nanoparticle size. The reduction in the nanoparticle size in the presence of hexacyanoferrate(HI) ions is proposed to result from the dissolution of surface Pt atoms through complexation with the strong cyanide ligand. Thiosulfate ions bind to the nanoparticle surface and act as a capping material, resulting in the stability of the nanoparticles. Judging from these observations, it is possible that the mechanism of this catalytic reaction involves the thiosulfate ions binding to the free sites on the surface of the nanoparticles, followed by reaction with hexacyanoferrate ions approaching the nanoparticle surface from the solution. Conducting the reaction with the nanoparticles preexposed to thiosulfate results in very little change in the centers and widths of the size distributions of the nanoparticles, thus suggesting that thiosulfate ions bind to the nanoparticle surface and inhibit desorption of Pt atoms by hexacyanoferrate(III) ions. The kinetics of the electron-transfer reaction during the first and second cycles is similar. The activation energy of the nanoparticle catalytic reaction is found to decrease linearly with increasing nanoparticle concentration during both the first and second cycles. If increasing the nanoparticle concentration leads to more aggregation, then these results suggest that the aggregated Pt has greater catalytic activity than the individual nanoparticles.