The electrochemistry of horse heart cytochrome c has been investigated at conventionally sized graphite substrates and carbon microdisk electrodes. The unusual concentration dependence of the voltammetric results at both the macro and micro-sized electrodes may be explained by a model which assumes that self-inhibition of part of the electrode surface occurs by adsorption of the protein itself, or constituents of the protein, thereby giving rise to a partially blocked electrode. Under conditions where the fraction of the electrode blocked is high, the electrochemical behavior is influenced strongly by the non-linear mass transport to the remaining μm- to sub- μm-sized active sites of the electrode surface. After allowing for the kinetic effects of the non-linear diffusion, an exceptionally high value of ko > 0.4 cm s-1 is obtained for the standard heterogeneous charge transfer rate constant. This value is considerably higher than values published previously which have been based on calculations where the mass transport has been assumed to occur solely by linear diffusion to a fully electroactive surface.