Heme-substituted horseradish peroxidases and myoglobins were reconstituted from the apoenzyme using mesoheme and diacetyldeuteroheme. X-ray absorption spectroscopy was used to determine the dimensions of the active sites of these heme-substituted proteins, and were compared with those of the proto-hemeproteins. The change in the active-site structure corresponded with the electron withdrawing and donating effects of the different side chains. The oxidation-reduction potentials of Fe4+/Fe3+ couples of the heme-substituted proteins were measured at pH 7 with K2IrC16. The oxidation-reduction potential sequence for compound I/compound II was diacetyldeutero-> proto-> meso-in horseradish peroxidase. The oxidation-reduction potential sequence for compound II /ferric was meso-> proto-> diacetyldeutero-in both HRP and myoglobin. These results indicate that the oxidation of ferric to ferryl form may be related to a radical mechanism. A net charge theory was also proposed to explain these results.