Epoxidation of cis-1-propenylphosphonic acid (CPPA) was carried out by using Na2WO4 or Na2MoO4 as catalyst, 30% hydrogen peroxide as an oxidant in H2O. Kinetic study showed that the epoxidation was zero-order both in H2O2 and in CPPA with perfect linearity. The reaction was first-order only in catalysts. IR spectra showed that the active peroxo group in the epoxidation was a metal-dioxygen ring. The characteristic absorption peaks were 856 and 866 cm(-1) for Mo- and W-dioxygen rings, respectively. A mechanism of the epoxidation was suggested and discussed. The pH value of reaction solution and the negative charge on the carbon atoms of double bonds play an important role because they could influence the concentration of active complexes and the rate of insertion reaction. The research indicated that not only in non-protic solvents but also in H2O, the active peroxo groups were the metal-dioxygen rings for the epoxidation of olefins catalyzed by W(VI) or Mo(VI) complexes. (C) 2003 Elsevier B.V. All rights reserved.