The Pt(100)-CO interaction in aqueous electrolytes was examined by using rotating disk methods in combination with in-situ surface X-ray scattering (SXS) measurements. The analysis of the SXS results indicates that the topmost platinum atoms expand away from the second layer by ca. 4% when H-upd was completely displaced from Pt(100) by CO to form a saturated layer of CO. Assuming that gas-phase heats of adsorption for CO apply as well to the liquid-solid interface, we estimate that the Gibbs energy change for the displacement of Hupd by CO On Pt(100) is close to -90 kJ/mol. A Pt(100)-CO surface normal interlayer spacing of 1.4 +/- 0.4 Angstrom was extracted from SXS measurements, suggesting that CO is adsorbed primarily at the 2-fold bridge-bonded sites, or possibly a mixture of bridge and atop sites. In contrast to the Pt(111)-CO system, no structures of COad with long-range order were formed on Pt(100). Two different forms of COad are formed at the Pt(100)-electrolyte interface: the weakly adsorbed state which is oxidized in the pre-ignition potential region, and the strongly adsorbed state which is oxidized in the ignition potential region. Although the nature of COad is different before and after the ignition potential, we proposed that the mechanism for CO oxidation on Pt(100) is the same in both the pre-ignition and ignition potential regions, e.g., adsorbed CO reacts with hydroxyl species (OHad) through a Langmuir-Hinshelwood type reaction. The kinetics of CO oxidation on Pt(hkl) surfaces is found to vary with crystal face. The difference in activity is attributed to the structure-sensitive adsorption of CO, OHad, and anions from the supporting electrolytes.