The oxiation of benzoin in non-aqueous medium catalyzed by cobalt complexes has been investigated under various reaction conditions. The effects of cobalt(II) salts used as catalysts, solvents, and some additives such as imines and KOH have been studied. The catalytic activity of benzoin oxidation has been found to depend appreciably on these factors. A small amount of benzoic acid, the CC cleavage oxidation product, in addition to the main product, benzil, has been obtained by using Co(II)(acac) 2(bis(acetyl- acetonato)cobalt(II)) as a catalyst, instead of simple Co(II) salts such as Co(II) nitrate or Co(II) chloride. The oxiation of benzoin in acetonitrile catalyzed with Co(II)(acac) 2 is first order in benzoin, second order in the catalyst and independent of the partial pressure of oxygen. The reaction process of the Co(II)(acac) 2 catalyzed oxidation of benzoin was followed by electronic absorption and NMR spectra, during which process it has been found that Co(II) species have been oxidized to Co(III) species in the presence of both benzoin and oxygen. From the rate law and the spectroscopic data obtained, it was suggested that the binuclear Co(II) species, which are present in equilibrium with Co(II)(acac) 2 monomer in acetonitrile solvent, and benzoin are in equilibrium with a binuclear cobalt—benzoin complex which in the rate-determining step loses a proton, followed by the rapid interaction with oxygen to form benzil.