The electron transfer (ET) between cytochrome c (Cyt c) in water (W) and 1,1'-dimethylferrocene (DiMFc) in 1,2-dichloroethane (DCE) was studied. The cyclic voltammograms obtained for the interfacial ET under various conditions could be well reproduced by digital simulation based on the ion-transfer (IT) mechanism, in which the ET process occurs not at the DCE/W interface but in the W phase nearest the interface. In this mechanism, the current signal is due to the IT of DiMFc(+) as the reaction product. On the other hand, the measurement of the double-layer capacity showed that Cyt c is adsorbed at the DCE/W interface. However, the contribution from the adsorbed proteins to the overall ET is considered to be small because of the thicker reaction layer in the IT mechanism. These findings would offer a useful suggestion for the behaviors of Cyt c in vivo.