The kinetics of the reaction of neutral [Mo(S2C6H4)(3)] with hydrogen sulfite to produce the anionic Mo(V) complex, [Mo(S2C6H4)(3)](-), and sulfate have been investigated, It has been shown that [Mo(S2C6H4)(3)] acts as the electron-proton sink in the oxygenation reaction of HSO3-by water. Reaction rates, monitored by UV/vis stopped-flow spectrometry, were studied in THF/water media as a function of the concentration of HSO3- and molybdenum complex, pH, ionic strength, and temperature. The reaction exhibits pH-dependent HSO3- saturation kinetics, and it is first-order in complex concentration. The kinetic data and MS-ESI spectra are consistent with the formation of [MO O(S2C6H4)(2)(S2C6H5)](-) (1) adduct as a crucial intermediate that transfers the oxygen atom to HSO3- yielding the Mo(V) species quantitatively.