A 4-nitrothiophenol (4-NTP, Ar-NO2) modified electrode was prepared by dipping a gold electrode into a 4-NTP/dichloromethane solution (self-assembly method). The redox response of the electrode depended on the reduction stages of the nitro group in the 4-NTP molecule. When the 4-NTP on the electrode was partially reduced to 4-hydroxylaminothiophenol (Ar-NHOH) at -0.2 to -0.3 V vs SCE (the potential depends on the pH of the electrolyte solution), the electrode showed a pair of redox responses in an acidic aqueous solution. The responses are attributed to the redox cycling between 4-nitrosothiophenol (Ar-NO) and Ar-NHOH with two-electron and two-proton transfer.