Keggin type polyoxometalates can be used as catalysts for the activation of 30% aqueous hydrogen peroxide for the selective transformation of various organic substrates. In this paper, results are presented for the oxidation of sulfides to sulfoxides in high to excellent yields using 30% aqueous H2O2 as oxidant and [PZnMo2W9O39](5-) as catalyst in acetonitrile.