This invention relates to a Polyoxometalate (POM) represented by the formula: (A.sub.n).sup.m+[H.sub.qM.sub.16X.sub.8W.sub.48O.sub.184(OH).sub- .32].sup.m- or solvates thereof, wherein: A represents a cation, n is the number of the cations A, m is the charge of the polyoxoanion, q is the number of protons and varies from 0 to 12, M represents a transition metal, and X represents a heteroatom selected from P, As and mixtures thereof. This invention also relates to a process to produce such POMs and to a process for the homogeneous or heterogeneous oxidation of organic substrates comprising contacting the organic substrate with such POMs.