The kinetics of (salen)Mn-III complexes-catalysed oxidation of aryl methyl sulfoxides with hydrogen peroxide in 80% acetonitrile-20% water solvent mixture at 25 degrees C has been followed spectrophotometrically. The reaction is first-order in (salen)Mn-III, zero-order in hydrogen peroxide and fractional-order in sulfoxide. Also, it has been found that nitrogenous bases affect the oxidation, while free-radical inhibitor does not. However, an increase in the water content of the solvent mixture causes an increase in the rate Of reaction. Stoichiometry between H2O2 and sulfoxide has been found to be 1:1 and the product analysis confirms the formation of sulfone and the regeneration of (salen)Mn-III complex. These observations have been well analyzed in favor of a Michaelis-Menten type mechanism, involving a manganese(III)-hydroperoxide complex as the reactive species. Using the derived rate law, the oxidant-substrate complex formation constant, K and the oxidant-substrate complex decomposition rate constant, k(2) have been evaluated. The proposed mechanism has been well Supported by electronic-oxidant and electronic-substrate effect Studies. (C) 2009 Elsevier B.V. All rights reserved.