The new binuclear oxothiolatorhenium(V) compound, Re 2 O 2 (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 μ-oxo species. The rate constants were determined for both steps in this reaction. The μ-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.