The invention relates to polyoxometalates represented by the formula (A.sub.n).sup.m+[R.sub.2(H.sub.2O).sub.6X.sub.2W.sub.20O.sub.70].sup.m- or solvates thereof, wherein A represents a cation, n is the number of the cations, m is the charge of the polyoxoanion, and X represents a heteroatom selected from Sb.sup.III, Bi.sup.III, As.sup.III, Se.sup.IV and Te.sup.IV, a process for their preparation and their use for the catalytic oxidation of organic molecules.