Electrochemistry of cytochrome c in a gamma-alumina membrane with pierced micropores (diameter : 0.15 and 0.03 mu m) has been examined. Reversible electrochemical reaction was observed at glassy carbon electrodes covered with either gamma-alumina membrane. A slightly positively charged gamma-alumina wall was considered to be effective on such an orientation of cytochrome c that enabled easy electron exchange at the gamma-alumina wall/solution interface near the electrode surface. Tn the micropores, it was observed that cytochrome c was concentrated owing to monolayer adsorption onto the alumina wall. Macroscopically, a linear diffusion model was applicable to the present system. Thus, the gamma-alumina membrane was confirmed to act as a "solid-state" promoter in the electrochemical reaction of cytochrome c. Microscopic treatment suggested that the diffusion mechanism of cytochrome c on the alumina wall is not electron hopping but physical displacement.