Methylrhenium dioxide, CH3ReO2 (or MDO), is produced from methylrhenium trioxide, CH3ReO3 (or MTO), and hypophosphorous acid in acidic aqueous medium. Its mechanism is discussed in light of MTO’s coordination ability and the inverse kinetic isotope effect (kie) : H2P(O)OH, k = 0.028 L mol(-1) s(-1); D2P(O)OH, k = 0.039 L mol(-1) s(-1). The Re(V) complex, MDO, reduces perchlorate and other inorganic oxoanions (XO(n)(-), where X = Cl, Br, or I and n = 4 or 3). The rate is controlled by the first oxygen abstraction from perchlorate to give chlorate, with a second-order rate constant at pH 0 and 25 degrees C of 7.3 L mol(-1) s(-1). Organic oxygen-donors such as sulfoxides and pyridine N-oxides oxidize MDO to MTO as do metal oxo complexes : VO(aq)(2+), VO2(aq)(+), HOMoO2 (aq), and MnO4-. The reaction between V-(aq)(2+), With MTO and the reduction of VO2+ with MDO made it possible to determine the free energy for MDO/MTO. Oxygen-atom transfer from oxygen-donors to MDO involves nucleophilic attack of X-O on the electrophilic Re(V) center of MDO; the reaction proceeds via an [MDO . XO] adduct, which is supported by the saturation kinetics observed for some. The parameters that control and facilitate the kinetics of such oxygen-transfer processes are suggested and include the force constant for the asymmetric stretching of the element-oxygen bond.