Using a combination of density functional calculations and Mossbauer spectroscopy, we have examined chloroperoxidase compound II ( CPO-II). The Mossbauer spectrum of CPO-II suggests the presence of two distinct ferryl species in an similar to 70: 30 ratio. Density functional calculations and cryogenic reduction and annealing experiments allow us to assign the major species as an Fe(IV) OH intermediate. The Mossbauer parameters of the minor component are indicative of an authentic iron(IV) oxo species, but we have found the 70: 30 ratio to be pH invariant. The unchanging ratio of component concentrations is in agreement with CPO-II's visible absorption spectrum, which shows no change over the enzyme's range of pH stability.