In this study, we measured the contributions of the ionization of the heme propionates to the reduction potentials of heme b and heme a (bis)N-methylimidazole complexes in various low-dielectric constant conditions. Additionally, we measured the effects of H-bond to the heme a formyl group on the reduction potential of the heme. The performed electrochemical measurements show that ionization of the heme propionates lead to the largest redox change in dichloromethane with no electrolyte. The measured reduction potential changes for heme b and heme a were -55 and -47 mV (+/-10 mV) per ionized propionate, respectively. For heme a, the study demonstrates how the dielectric constant of the medium is important in the magnification of the Delta pK(a) upon redox-linked ionization of the heme propionates and their roles in the proton pump of cytochrome c oxidase.