We report results on the large catalytic effect of spherical gold nanoparticles on the rate of reduction of hexacyanoferrate (III) by sodium borohydride in aqueous solution. Because the gold nanoparticles remain stable and no aggregation takes place during the reaction, it can be monitored until completion. The presence of colloidal gold leads to a considerable increase in the observed reaction rate and to a change in the order of reaction. The reaction is first-order with respect to the hexacyanoferrate (III) concentration and gold particle concentration, but the reaction order with respect to borohydride ion is more complex. The activation energy is found to be 15 kJ/mol for 15 nm gold particles. The redox reaction is activation-controlled under most conditions, but the rate of reaction approaches the diffusion limit for higher borohydride concentrations and is over 10(4) times faster than in the absence of the gold catalyst.