The adsorption state of heptyl viologen (HV) on the Au(111) electrode surface was studied by in situ infrared reflection absorption spectroscopy (IRAS). Both the dication (HV2+) and monocation (HV+) species were detected, and they indicated complicated spectral changes with the electrode potential. The spectral assignment was carried out with the aid of theoretical calculation utilizing the density functional theory (DFT) method. It was indicated that the HV2+ molecule adsorbs in a side-on orientation, being reduced to form a face-on dimer composed of two monocations at approximately + 180 mV versus Ag \ AgCl where the current spikes appear on the cyclic voltammograms (CVs). (C) 2002 Elsevier Science B.V. All rights reserved.