Combined CO-stripping cyclic voltammetry and FT-IR spectroscopy measurements have allowed us to monitor changes in the coverage and structure of CO adlayers on Pt(1 1 1) electrodes in 0.1 M H2SO4 as a function of potential. Our results show that in CO-free solutions the maximum coverage is theta(CO) = 0.68 and that higher coverages can only be achieved in the presence of CO in the solution. Saturation coverages can only be reached if the potential at which the electrode is held during CO adsorption (dosing potential, E-d) is more negative than 0.30 V vs. RHE. The lowest CO coverage at which hydrogen adsorption on the Pt(1 1 1) electrode is completely blocked is theta(CO) = 0.63, which corresponds to an E-d = 0.50 V vs. RHE. Our results suggest that the process at the pre-peak (and, hence, the oxidation at low overpotentials of bulk CO in CO-saturated solutions), corresponds to the oxidation of adsorbed CO by reaction with oxygenated species nucleating at steps, the main peak appearing when nucleation of oxygenated species at the terraces also occurs. (c) 2005 Elsevier B.V. All rights reserved.