Quasi-reversible and direct electron transfer was observed between an iodide-modified Au electrode and cytochrome c, as well as between cytochrome c in an iodide-containing solution and a bare Au electrode. The results suggest that an electrostatic interaction between cytochrome c and the iodide-modified electrode surface plays an important role in the electrochemical response. A mechanism is proposed which involves adsorption of cytochrome c on the surface of the iodide-modified electrode, followed by rapid desorption after the electron transfer process.