The resonance Raman spectra of the hydroperoxo complex of camphor-bound CYP101 have been obtained by cryoradiolytic reduction of the oxygenated ferrous form that had been rapidly frozen in water/glycerol frozen solution; EPR spectroscopy was employed to confirm the identity of the trapped intermediate. The V(O-O) mode, appearing at 799 cm(-1), is observed for the first time in a peroxo-heme adduct. It is assigned unambiguously by employing isotopomeric mixtures of oxygen gas containing 50% (OO)-O-16-O-18, confirming the presence of an intact O-O fragment. The V(Fe-O) mode is observed at 559 cm(-1) (H2O). Furthermore, both modes shift down by 3 cm(-1), documenting the formulation as a hydroperoxo complex, in agreement with EPR data.