The electron exchange reaction of cytochrome c with NADPH-cytochrome P450 reductase was investigated electrochemically. For this purpose the electrochemical behavior of cytochrome c in solution, adsorbed or covalently immobilized on a modified gold electrode surface was studied. In the case when cytochrome c was in solution or only electrostatically adsorbed on the electrode surface, fast electron transfer was observed with NADPH-cytochrome P450 reductase. When cytochrome c was covalently bound to the electrode surface, in spite of quasi-reversible electron exchange with the electrode, no electron transfer was observed with NADPH-cytochrome P450 reductase. These results suggest that electrostatically adsorbed cytochrome c on the modified electrode surface has some mobility that allows re-orientation as required to interact both with the electrode and with the NADPH-cytochrome P450 reductase. In contrast, covalent binding of cytochrome c leads to permanent orientation towards the electrode surface and thus blocks the cytochrome c electron accepting site from the reductase.