We have investigated the structure of the CO adlayer produced through methanol dissociative adsorption on Pt(100) in acidic solution by in situ IR-visible sum frequency generation (SFG) vibrational spectroscopy. Our results provide evidence of a site conversion from bridging CO to atop CO as the electrode potential varies from 350 to 450 mV versus Ag\AgCl\KClsat. A new broad band is detected above 450 mV This spectral feature is tentatively assigned to an electronic excitation at the sum frequency between the metal Fermi level and unoccupied electronic states of OHads. The CO site migration could be triggered by OHads formation on terraces, the latter process perturbing the balance between CO-CO and CO-H2O lateral dipolar interactions. (C) 2004 Elsevier Ltd. All rights reserved.