A family of organic sulfides was oxidized to the corresponding sulfoxides by hydrogen peroxide. Such reactions are, however, very slow and meaningless in practice without an effective catalyst. The oxidation was successfully catalyzed by CH3ReO3, a water-soluble organometallic oxide. A kinetic study was carried out in 1:1 (v/v) acetonitrile-water at pH 1 and at 25 degrees C. The kinetics can be resolved into two steps. First, H2O2 and CH3ReO3 react to form 1:1 and 2:1 rhenium peroxides, denoted as A and B, respectively. In the second step A and B react with the substrate forming the product. The rate constants for the various steps of these reactions were evaluated using steady-state techniques and are on the order of 10(3) L mol(-1) s(-1) for aryl methyl sulfides and 10(4) L mol(-1) s(-1) for dialkyl sulfides. Both A and B are reactive, A moreso than B. The kinetic results point to a mechanism that involves the nucleophilic attack of the sulfur atom on a peroxide oxygen of the rhenium peroxides. This formulation is consistent with the accelerating effects of electron-donating substituents.