A significant effort has been done towards the development of new catalysts for the synthesis of epoxides in a more environmental friendly way. We have found that the mono oxovanadium(IV) complexes of the N2O2 donor set ligands are able to catalyze selective epoxidation of cyclooctene with high yield using O-2 in polar solvents. In general, catalytic activity increases by decreases of the number of electron -donating groups and the catalytic selectivity varies according to the types of substituents in the ligands. The vanadium complexes with strongly donor ligands, are more stable during the oxidation process. The conversion of cyclooctene increases with the E-redox(0')/Delta EP ratio of V5+/V4+ couples. (c) 2006 Elsevier B.V. All rights reserved.