We report the study of methanol electro-oxidation on Pt(110) using infrared-visible sum-frequency generation (SFG) vibrational spectroscopy. The use of this technique enables to probe the vibrational and electronic properties of the interface simultaneously in situ. We have investigated the vibrational properties of the interface in the COads, internal stretch spectral region (1700-2150 cm(-1)) over a wide range of potentials. The analysis of the evolution of the C-O stretch line shape, which is related to the interference between the vibrational and electronic parts of the non-linear response, with the potential allows us to show that the onset of bulk methanol, oxidation corresponds to the transition from a negatively to a positively charged surface. (C) 2005 Elsevier B.V. All rights reserved.