The invention relates to polyoxometalates represented by the formula (A.sub.n).sup.m+ [M.sub.y(H.sub.2O).sub.(p.y)X.sub.2W.sub.22O.sub.74(OH).sub.2].sup.m- or solvates or mixtures thereof, wherein A represents a cation, n is the number of the cations, m is the charge of the polyoxoanion, M represents a transition metal selected from Pd, Pt and mixtures thereof, y is 1, 2, 3 or 4, p is the number of water molecules bound to one M and varies from 3 to 5, 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.