The electrochemical properties of Fe(II) complexes with a number of cysteine-containing oligopeptides were investigated in aqueous Triton X-100 (10%) micelle solution. The complexes with an aromatic ring such as [Fe(Z-cys-Pro-Leu-cys-Gly-NH-C6H4-p -F)(2)](2-), [Fe(Z-cys-Pro-Leu-Cys-Gly-NH-CH2-CH2-C6H4-p-F)(2)](2-) and [Fe(Z-cys-Pro-Leu-cys-Gly-Phe-OMe)(2)](2-) exhibit their redox potentials at -0.28, -0.30 and -0.26 V vs. SCE, respectively, in aqueous Triton X-100 (10%) micelle solution with good reversibility. The potentials are very close to that of native rubredoxin (-0.31 V vs. SCE in aqueous solution). A para-substituent effect was observed for the redox potentials of [Fe(Z-cys-Pro-Leu-cys-Gly-NH-C6H4-p-X)(2)](2-) (X = OMe, H, F and CN) in aqueous micelle solution. The most positively shifted potential was found at -0.26 V vs. SCE for the complex with the strongest electron-withdrawing cyano (CN) substituent. The results show that the complexes with an aromatic ring are quite stable in such aqueous micelle solutions.