Kinetic studies on the catalytic activity of phenylphosphopolyperoxotungstate complexes [C21H37N](2)[(PhPO3){WO(O-2)(2)](2){WO(O-2)(2)H2O}] (A) and IC21 H37N](2)[(p-NO2Ph(O)PO3){WO(O-2)(2)}(2){WO(O-2)(2)H2O}], (B) toward oxidation of organic sulfides by hydrogen peroxide were carried out in CH3OH/H2O mixed solvents. Compound B has been prepared for the first time as a modified form of A. The presence of a nitro group in B increases the electrophilicity of the catalyst, and leads to a higher activity and selectivity for the oxidation of sulfides to sulfoxides by H2O2. The reaction rate is found to be first-order in the [catalyst], the [sulfide] and the [H2O2] at low [H2O2] (<0.01). At higher [H2O2] (>0.06M), the rate becomes almost zero-order in [H2O2]. The rate of oxidation increases with the sulfide nucleophilicity. Hammett correlations of the rate constants with or for different substituted diaryl sulfides give negative reaction constants (p similar to - 1.2). The effects of temperature variation, the ratio of water in the solvent, and the solution acidity on the reaction rate are also investigated. A three-membered-ring transition state has been proposed, in which the sulfide attacks the electrophilic O-atom of the W-peroxo species prior to a complete oxygen transfer, as a key step in the reaction mechanism. A radical mechanism has been ruled out. (c) 2007 Elsevier B.V. All rights reserved.