Spectroelectrochemical experiments and density functional theory (DFT) based calculations are combined to elucidate the nature of the species adsorbed when a perchloric acid solution saturated with carbon dioxide is in contact with well-defined electrode surfaces. Previous results reported for the Pt(111) electrode are revised and completed with new data for a palladium monolayer deposited on the Pt(111) electrode. From the DFT calculations and from the potential and pH dependent behavior of the main adsorbate vibrational bands, it is suggested that, for potentials below 0.65 V, bicarbonate species are adsorbed at the Pt(111) electrode surface in a short-bridge bidentate configuration. At higher potentials, the adsorbate adlayer is completed by the formation of adsorbed carbonate also in a short-bridge configuration. The ratio between the surface coverages of adsorbed bicarbonate and carbonate depends on both the electrode potential and the solution pH. Adsorbed bicarbonate prevails at the palladium monolayer irrespective of the electrode potential. This behavior, together with a concomitant lower adsorbate coverage, can be related to the competitive adsorption of hydroxylated species.