The reaction of peroxotungstates (H2WO4 + H2O2) with H2SeO4 gave the novel selenium-containing dinuclear tungsten species, (TBA)(2)[SeO4{WO(O-2)(2)}(2)] (I, TBA = [(n-C4H9)(4)N](+)), which was characterized by elemental analysis, IR, Raman, UV-vis, Se-77 NMR, W-183 NMR, and CSI-MS. Various kinds of homoallylic and allylic alcohols were efficiently epoxidized to the corresponding epoxy alcohols in high yields with 1 equiv. H2O2 with respect to the substrates. Compound I showed the highest catalytic activity for H2O2-based epoxidation of homoallylic and allylic alcohols among selenium and tungsten complexes. The turnover frequency reached up to 150 h(-1) in a 10 mmol-scale epoxidation of cis-3-hexen-1-ol and this value was the highest among those reported for the transition-metal catalyzed epoxidation of homoallylic alcohols with H2O2. The kinetic, mechanistic, computational studies showed that the stabilization of the transition-state by the hydrogen bonding between I and the substrates results in the high reactivity for the I-catalyzed epoxidation of homoallylic and allylic alcohols. The nature of the hetero atoms in the di- and tetranuclear peroxotungstates with XO4n- ligands (X = As(V), P(V), S(VI), Si(IV), etc.) was crucial in controlling the Lewis acidity of the peroxotungstates, which significantly affects their electrophilic oxygen transfer reactivity. All the data of the structural, kinetic, spectroscopic, and computational comparison show that the dimeric peroxotungstate unit, {WO(O-2)(2)}(2), in I is activated by the SeO42- ligand.