The dioxo tungsten(VI) and molybdenum(VI) complexes WCl2(O)(2)(OPMePh2)(2), WCl2(O)(2)dppmO(2), and MoCl2(O)(2)dppmO(2), the oxoperoxo compounds WCl2(O)(O-2)(OPMePh2)(2), WCl2(O)(O-2) dppmO(2), and MoCl2(O)(O-2) dppmO(2), and the oxodiperoxo complexes, W(O)(O-2) (2)dppmO(2) and Mo(O)(O-2)(2)dppmO(2) have been prepared and characterized by IR spectroscopy, P-31 NMR spectroscopy, elemental analysis, and X-ray crystallography. The structural and X-ray crystallographic data of compounds WCl2(O)(2)(OPMePh2)(2), WCl2(O)(O-2)(OPMePh2)(2), MoCl2(O)(2)dppmO(2), C4H10O, WCl2(O)(2)dppmO(2), Mo(O)(O-2)(2)dppmO(2), and W(O)(O-2)(2)dppmO(2) are also detailed. All complexes were studied as catalysts for cis-cyclooctene epoxidation in the presence of tert-butyl hydroperoxide (TBHP) or H2O2 as an oxidant. The Mo-based catalysts showed a superior reactivity over W-based catalysts in the TBHP system. On the other hand, in the H2O2 system, the W-based catalysts (accomplishing nearly 100% epoxidation of cyclooctene in 6 h) are more reactive than the Mo catalysts (< 45% under some conditions). Various solvent systems have been investigated, and ethanol is the most suitable solvent for the H2O2 system.