The new binuclear oxothiolatorhenium(V) compound, Re2O2(mtp)(3), D-1, mtpH(2) = 2-(mercaptomethyl)thiophenol. was found to activate molecular oxygen. which has no precedent in oxorhenium catalysis. The kinetics and mechanism of the oxidation of triaryl phosphines and methyl phenyl phosphines by O-2 were investigated. The reaction pathway inferred from the kinetic data involves two Re(VII) intermediates, a peroxo and a mu -oxo species. The rate constants were determined for both steps in this reaction. The mu -oxo Re(VII) intermediate further reacts with a second phosphine forming more phosphine oxide as D-1 is recovered. This step is very rapid, and does not affect the kinetics. Other substrates, taken alone, such as sulfides and dienes, which coordinate to D-1 weakly, or do not coordinate, were not oxidized by molecular oxygen. However, in the presence of a small amount of phosphine to open up the sulfur bridge in D-1, then sulfides and dienes were also oxidized to sulfoxides and epoxides accordingly. The relative reactivities of the substrates fall in the order: phosphines > sulfides > dienes.