The invention relates to supported polyoxometalates represented by the formula (A.sub.n).sup.m+ [M.sub.4(H.sub.2O).sub.10(XW.sub.9O.sub.33).sub.2].sup.m- or solvates thereof, wherein A represents a cation, n is the number of cations, m is the charge of the polyoxoanion, M is a transition metal, and X is an element selected from the group consisting of As, Sb, Bi, Se and Te, characterized in that the polyoxometalate is supported on a solid support selected from the group consisting of Al.sub.2O.sub.3, MgO, TiO.sub.2, ZrO.sub.2, SiO.sub.2, mesoporous silica, active carbon, diatomite, clays, zeolites, polyoxometalate salts and mixtures thereof, with the proviso that the polyoxometalate salt supports are different from the supported polyoxometalates defined by the above formula, a process for their preparation and their use for the catalytic oxidation of organic molecules.