MCD spectroscopy has been used to probe the ground and excited state electronic structure of [(TMP)Fe(IV)=O](+), in order to obtain additional insight into the electronic structure of enzymic compound I catalytic intermediates. Differences between the MCD of HRP I and the model parallel differences in the exchange interaction. Variable-temperature MCD has determined the magnitude of the ground state zero-field splitting (D-3/2 = +-5.3 cm(-1)), which reflects a small energy separation (triangle) between the Fe d(xy) and d(xz,yz) orbitals due to Fe dxz,yz --> porphyrin p pi* back-bonding interactions.