A microscopic model of electron transfer, which assumes that electron transfer occurs only after radial diffusion to oxygen-functionalized sites of molecular dimension in size, has been developed to explain the electrochemical reduction of horse heart cytochrome c at graphite electrodes. By use of this model, the conclusion is reached that the rate of electron transfer is greater than 1 cm s-1 at both basal- and edge-plan graphite electrodes and silanized or "aged" graphite electrodes. According to the microscopic model, differences in shapes of cyclic voltammograms observed at basal-plane, edge-plane, and silanized or aged elctrodes are a result of different densities of electroactive sites and not variations in the heterogeneous rate of electron transfer as proposed previously based on the use of the macroscopic model which assumes that mass transport occurs via linear diffusion. The basal-plane electrode or silanized or aged adge-plane electrodes have a low surface density of oxygen-functionalized electroactive sites, and therefore radial diffusion is the dominant mode of mass transport. Sigmoidal-shaped voltammograms, corresponding to a reversible one-electron reduction process, are observed under these conditions. In contrast, edge-plane (high oxygen content) or polished basal-plane (medium oxygen content) have a higher density of electroactive sites. Under these conditions, diffusion layers overlap, thus destroying the radial diffusion terms. Peak-shaped curves are observed when linear, rather than radial, diffusion becomes the dominant mode of mass transport. This is the case at edge-plane and polished basal-plane electrodes. However, the peak-shaped curves still correspond to a reversible electron-transfer step. The concept that the heterogeneous rate of electron transfer to oxygen-functionalized electroactive sites is extremely fast for electrochemical reduction of cytochrome c at graphite electrodes is consistent with the fast homogeneous rates of chemical redox reactions that have been reported for cytochrome c. The microscopic model of reversible (fast) electron transfer rationalizes existing electrochemical data available for reduction of cytochrome c at graphite electrodes in a superior way to the classical macroscopic model in which variable rates of quasi-reversible electron transfer occur at electroactive sites having a relatively large surface area.