Modeling studies suggest that electrons are transferred from cytochrome c to cytochrome c peroxidase (CcP) with cytochrome c predominantly bound at a site facing the gamma-meso edge of the CcP prosthetic heme group (Poulos, T.L., and Kraut, J. (1980) J. Biol. Chem. 255, 10322-10330). As shown here, guaiacol and ferrocyanide are oxidized at a different site of CcP. Thus, the oxidations of cytochrome c and guaiacol are differentially inactivated by phenylhydrazine and sodium azide. The loss of guaiacol oxidation activity correlates with covalent binding of 1 equivalent of [14C]phenylhydrazine to the protein, whereas the slower loss of cytochrome c activity correlates with the appearance of a 428-nm absorbance maximum attributed to the formation of a sigma-phenyl-iron heme complex. The delta-meso-phenyl and 8-hydroxymethyl derivatives of heme are formed as minor products. Catalytic oxidation of azide to the azidyl radical results in inactivation of CcP and formation of delta-meso-azidoheme. Reconstitution of apo-CcP with delta-meso-azido-, -ethyl-, and -(2-phenylethyl)heme yields holoproteins that give compound I species with H2O2 and exhibit 80, 59, and 31%, respectively, of the control kcat value for cytochrome c oxidation but little or no guaiacol or ferrocyanide oxidizing activity. Conversely, CcP reconstituted with gamma-meso-ethylheme is fully active in the oxidation of guaiacol and ferrocyanide but only retains 27% of the cytochrome c oxidizing activity. These results indicate that guaiacol and ferrocyanide are primarily oxidized near the delta-meso-heme edge rather than, like cytochrome c, at a surface site facing the gamma-meso edge.