The oxygenation of sulfides to the corresponding sulfoxides catalyzed by H5PV2Mo10O40 and other acidic vanado-molybdates has been shown to proceed by a low-temperature electron transfer-oxygen transfer (ET-OT) mechanism. First, a sulfide reacts with H5PV2Mo10O40 to yield a cation radical-reduced polyoxometalate ion pair, R-2(+center dot),(H5PVVMo10O40)-V-IV-Mo-V, that was identified by UV-vis spectroscopy (absorptions at 650 and 887 nm for PhSMe+center dot and (H5PVVMo10O40)-V-IV-Mo-V) and EPR spectroscopy (quintet at g = 2.0079, A = 1.34 G for the thianthrene cation radical and the typical eight-line spectrum for V-IV). Next, a precipitate is formed that shows by IR the incipient formation of the sulfoxide and by EPR a VO2+ moiety supported on the polyoxometalate. Dissolution of this precipitate releases the sulfoxide product. ET-OT oxidation of diethylsulfide yielded crystals containing [V(O)(OSEt2)(x)(solv)(5-x)](2+) cations and polyoxometalate anions. Under aerobic conditions, catalytic cycles can be realized with formation of mostly sulfoxide (90%) but also some disulfide (10%) via carbon-sulfide bond cleavage.