Kinetic studies on the catalytic activity of phenylphosphopolyperoxotungstate complexes [C21H37N](2)[(PhPO3)[WO(O-2)(2))(2){WO(O-2)(2)H2O}] (A) and [C21H37N](2)[(p-NO2Ph(O)PO3)[WO(O-2)(2)}(2) {WO(O-2)(2)H2O}] (B) in the epoxidation of olefins with hydrogen peroxide as the oxidizing agent were carried out in CH3CN/H2O solvent mixtures. The nitro-substituted catalyst, B, was shown to be more active than A. However, the higher acidity of B enhances more epoxide ring-opening to yield 1,2-diol. Under biphasic conditions, the epoxide ring-opening is reduced, and in most cases the 1,2-diol products do not form. Under homogeneous conditions, the reaction is first-order with respect to the catalyst and the olefin, and between zero- and first-order with respect to the H2O2 concentration. The rate of the reaction increases with the olefin nucleophilicity. The epoxidation rate constants of para and meta substituted styrenes are correlated with sigma(+) according to the Hammett equation. The reaction constants (p similar to -0.9) and the reaction stereoselectivity suggest a mechanism, which involves an external nucleophilic attack of the pi-system of the olefin on the electropositive oxygen of the W-peroxo group. A catalytic cycle involving a fast (pre-equilibrium) reaction between H2O2 and the catalyst followed by the oxygen-transfer step (rate-determining) is proposed. (c) 2008 Elsevier B.V. All rights reserved.