Adsorption of benzoic acid on an Au (100) electrode in 0.1 M HClO4 has been investigated by IRAS and voltammetry. In the IRAS, the vibrational band of COO- symmetrical stretch corresponding to the vertically adsorbed benzoate anion species were observed. At slightly higher frequency a new band also due to the symmetrical stretch of the benzoate anion, and overlapping with the original band, appeared at 0.8 V (RHE). In the voltammogram, three reversible couple were observed; a peak at 0.5 V corresponds to the appearance of the COO- band and a peak observed at 0.8 V corresponds to the appearance of the new band. This indicates that the new band should be associated with some adsorption behavior of the vertical type such as a change of the packing arrangement at that potential.