A range of heteropolyoxometalates catalyze the oxidation of thioether analogs of mustard (HD) to the corresponding sulfoxides by tert-butyl hydroperoxide (TBHP) rapidly at 25 degrees C, with selectivities as high as any seen in the literature for thioether oxidations. The strongly acidic complexes H3PW12O40 and H3PMo12O40 function primarily as acid catalysts for thioether oxidation by TBHP, producing sulfoxide with 98-99% selectivities (1-3% sulfone present) at modest conversions. Three lines of evidence involving the model compound, tetrahydrothiophene (THT), indicate that several vanadium-substituted heteropoly acids, including hydrated H5PV2Mo10O40 (1), are significantly more selective (>99.9%) : (1) No sulfone (THTO2) is detectable at the limits (1 part in 10(5)) of gas chromatographic and gas chromatographic-mass spectrometric analyses, (2) THTO and THTOH+ are both stable under the reaction conditions, and (3) (THTOH)(4)(H)(PV2Mo10O40), or 2, identified by NMR, IR, UV-visible, and elemental analysis, is stable for a period of at least 1 month under the conditions of the title catalytic reactions (homogeneous acetonitrile, 25 degrees C). The rates of thioether oxidation to sulfoxide catalyzed by 1, based on six aliphatic and aromatic thioethers, vary over a factor of more than 10(3) and do not correlate well with the thioether. redox potentials. The data, including rates and selectivities of the reaction with and without TBHP, establish that these reactions proceed by initial thioether oxidation and polyoxometalate reduction followed by reduced polyoxometalate reoxidation by TBHP. This mechanism is in sharp contrast to the conventional mechanisms for metal complex mediated peroxide oxidations (electrophilic peroxide activation by the metal complex or radical processes). Two important and heretofore undocumented roles of H2O are consistent with the rate studies of THT oxidation by 1 in the absence of TBHP. First, the reaction is strongly inhibited by water and the rate law is v(0) = k[THT][1][H2O](-1). In the absence of water, the rate becomes independent of THT substrate (v(0) = k’[1]) and dehydration of the polyoxometalate may be rate limiting. All the data, including the activation parameters (Delta H-double dagger = 8.06 +/- 0.64 kcal mol(-1) and Delta S-double dagger = 29.7 +/- 0.18 eu), are most consistent with outer sphere electron transfer for the key thioether oxidation step in the mechanism.