Adsorption of p-nitrobenzoic acid (PNBA) at an Au electrode in 0.1 M HClO4 solutions has been studied by combining cyclic voltammetry, electrochemical quartz crystal microbalance (EQCM), in situ Fourier transform infrared spectroscopy (FTIRS) and surface enhanced Raman spectroscopy (SERS). The adsorption of PNBA on Au electrode yields two current peaks appearing in the potential region between 0.3 and 1.0 V. A surface mass change of 74 ng cm(-2) has been determined by EQCM, corresponds to the adsorption, which associates to a variation of surface concentration of 2.8 x 10(14) molecules cm(-2). The spectral features observed from studies of in situ FTIRS and SERS illustrated, at molecular level, the potential-dependence properties of PNBA adsorption. The results demonstrated that the molecule of PNBA adsorbs in a Rat mode at potentials below 0.3 V and that at higher potentials it trends to adsorb in a vertical adsorption mode via its carboxylic group towards Au electrode surface.