Cationic sulfenatocobalt(III) complexes, with the remaining ligands amines, are deoxygenated upon reaction with hydrated methyldioxorhenium(V). The reactions of (Am)(5)Co-S(O)R2+ yield the thiolato complexes (Am)(5)Co-SR2+ and methyltrioxorhenium(VII), MTO. A kinetic study of these reactions was carried out, using the known reaction between MTO and hypophosphorous acid to prepare the Re(V) reagent in solution. The second-order rate constants between (Am)(5)Co-S(O)R2+ and MeReO2(aq), after correction for protonation, fall in the range 47-455 L mol(-1) s(-1) in aqueous solution at 25.0 degrees C and 1.0 M ionic strength. In the analysis of the pH effects, allowance was made for the formation of the protonated species (Am)(5)Co-S(OH)R3+ at high [H3O+]. The values of pK(a) are in the range 0.49-0.96, as determined from fitting the rate-pH profiles.