Horse heart cytochrome c serves as a test case for analyzing the role of temperature and pressure on electron transfer since its temperature- and pressure-dependent reduction potentials are known. Both the magnitude and direction of experimental reduction potential changes observed in cytochrome c as a function of temperature and pressure were reproduced empirically using a numerical solution to the Poisson-Boltzman equation. It is proposed based on these calculations and analysis of the X-ray crystal structures in both oxidation states that the heme iron is screened more efficiently from surface charges in the oxidized state.