The direct electrochemistry of cytochrome c, cytochrome b, and Cyt c/Cyt b(5) complex has been studied by cyclic voltammetry at a cysteine-modified gold electrode. The results indicate that this functional electrode permits selectively a quasi-reversible electrochemical response of cytochrome c only. The near-reversible electrochemical response of cytochrome b, was stimulated by Mg2+ ion. From titration experiments, we observed that either the Mg2+ ion or cytochrome b, could improve the cyclic voltammetric response of cytochrome c in different ways, and proved that the 1:1 protein-protein complex formed in the presence of Mg2+ ion even at higher ionic strength. Meanwhile, we observed that the Mg2+ ion and the ionic strength affected the electrochemical behaviour of cytochrome c more strongly than cytochrome b(5). Thus, we propose that there are different interaction models involving protein-electrolyte-electrode between cytochrome c, cytochrome b(5) and the 1:1 protein complex with the electrode. The mutual interaction between two proteins was favourable for the heterogeneous electron transfer reaction, and the assumed interaction patterns of the 1:1 protein complex with the electrode were also confirmed by potential-step chronocoulometry.