The pseudo-first order kinetics of oxidation of Mn(II) to Mn(III) in heteropoly ion, [SiMnIIW11O40H2]6-, was studied at 50-degrees-C using peroxydisulphate ion as an oxidizing agent. The pH was kept at 5.0 and ionic strength was 2.0 in Na2SO4. It was found that over a wide range of concentrations of S2O82- ion the kobs varies linearly with [S2O82-]1/2 rather than [S2O82-], values suggesting that oxidation of Mn(II) is preceeded by homolytic dissociation of S2O82- ion into SO4- ion radicals.