The reaction of the dinuclear peroxotungstate, [(n-C4H9)(4)N](2)[{WO(O-2)(2)}(2)(mu-O)] (II), with H2O2 gives the novel mu-eta(1):eta(1)-peroxo-bridging dinuclear tungsten species, [(n-C4H9)(4)N](2)[{WO(O-2)(2)}(2)(mu-O-2)] (I), which has been characterized by X-ray crystallography, elemental analysis, IR, Raman, UV-vis, and W-183 NMR. Only I is active for the epoxidation of cyclic, internal, and terminal olefins, whereas II is inactive for each. The low X-SO (X-SO = (nucleophilic oxidation)/(total oxidation)) value of I (0.18 +/- 0.02) in comparison with that of II (0.39 +/- 0.01) for the stoichiometric oxidation of thianthrene 5-oxide, which is a mechanistic probe for determining the electronic character of an oxidant, reveals that I is more electrophilic than II. On the basis of the kinetic and spectroscopic results, the catalytic epoxidation proceeds by the reaction of I with an olefin to form II and the corresponding epoxide followed by the regeneration of I by the reaction of II with H2O2.