Reactions of hydrogen peroxide with several lacunary polyoxometalates of the 1:11 series, XW(11)O(39)m(-) (X = Co3+, Ga3+, Fe3+, Si4+, and P5+), are reported. Synthetic pathways to new polyoxotungstates incorporating dioxygen moieties (peroxo and/or superoxo) are developed. The key step involves treating lacunary precursors with H2O2 in strongly buffered aqueous solutions. Upon reaction of H2O2 with alpha-[Co3+W11O39],(9-) (a) the central tetrahedral Co3+ is reduced to Co2+ acid (b) each of the four unshared oxygens surrounding the vacancy are replaced by a peroxide group, yielding salts of the tetraperoxide anion beta(3)-[(Co2+O4)W11O31(O-2)(4)](10-) (1) These results are unequivocally established by a combination of elemental analysis, spectroscopy (UV-Vis-near-IR and IR), magnetic moment determination, and complete X-Pay crystal structure analysis of (NH4)(9)K[(Co2+O4)W11O31 (O-2)(4)]H2O. The dioxygen O-O bonds are 1.41 and 1.44 Angstrom, typical of peroxo complexes. Salts of 1 are excellent stereoselective catalysts for the oxidation/epoxidation by H2O2. Reaction of 2-cyclohexenol with H2O2 catalyzed by 1 yields cis- and trans-2,3-epoxycyclohexen-1-ol (59.3% and 3.6%, respectively) acid 2-cyclohexen-1-one (28.3%). According to ESR and IR spectroscopic results, the reaction of H2O2 with other lacunary XW11O39m-anions (X = P5+, Si4+, Ga3+, and Fe3+) proceeds by a different mechanism which involves the loss of heteroatom and formation of an isopolytungstate containing superoxo moieties (g(1) = 2.039, g(2) = 2.014, g(3) = 2.009; v(0-0) = 1040 and 1060 cm(-1)).