The electro-oxidation of CO adlayer on Pt electrode in Cl--containing 0 1 M HClO4 has been investigated with in situ attenuated-total-reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) Two potentials were selected for predosing CO on the Pt electrode one is in the H-UPD region i e 0 1 V (vs RHE) and the other is in the double-layer region i e 0 45 V (vs RHE) The broadening of the pre-wave and the main peak for the CO oxidation is observed in addition to the positively shifted oxidation potentials The spectroelectrochemical data suggest the specific adsorption of Cl-starts at a potential as negative as 0 1 V which may compete with the adsorption of OH at CO-unoccupied sites (including but not limited to defect sites) and/or hinder the diffusion of CO to OH adsorption sites on Pt electrode slowing down the CO oxidation This competitive Cl-adsorption at lower potentials disrupts the interfacial free H2O structure on the top of CO adlayer signaled by a reduced OH stretching band intensity (C) 2010 Elsevier Ltd All rights reserved