As in previous work with Pt(111) electrodes, we have combined CO-stripping cyclic voltammetry, CO charge-displacement and FTIR spectroscopy measurements to determine changes in the coverage and structure of CO adlayers on Pt(100) electrodes in 0.1 M H2SO4 as a function of potential. In CO-free solutions the maximum coverage is theta(CO) = 0.79, which can only be reached if the potential at which the electrode is held during CO adsorption (dosing potential, E-d) is more negative than 0.25 V vs. RHE. Although the highest CO coverage of Pt(100) electrodes in contact with CO-saturated solutions could not be determined, our FT-IR results clearly show that, as in the case of Pt(111) electrodes, removing CO from the solution results in a partial, reversible desorption of the CO adlayer, and, hence, that the CO adlayer on the Pt(100) electrode is in equilibrium with the CO-containing solution. The lowest CO coverage at which hydrogen adsorption on the Pt(100) electrode is completely blocked is theta(CO) = 0.75, which corresponds to E-d = 0.40 V vs. RHE. The results reported here provide support to the hypothesis that the process at the pre-peak in CO-stripping voltammograms (and, hence, the oxidation at low overpotentials of bulk CO in CO-saturated solutions), corresponds to the oxidation of CO by reaction with oxygenated species nucleating at steps, the main CO-oxidation peak appearing when nucleation of oxygenated species at the terraces also occurs. (c) 2005 Elsevier B.V. All rights reserved.