In order to evaluate adsorption of CO species onto precious metal electrodes during electrochemical oxidation of methanol under hot aqueous conditions, adsorption of carbon monoxide (CO) on a Pt electrode was studied in hot aqueous solution of 0.005 M H2SO4-0.2 M Na2SO4 by a fast potential sweep method at temperatures ranging from 423 to 533 K under the corresponding vapor pressure. An electrochemical half cell fabricated in a pressure vessel was employed to realize electrochemical measurement under this high temperature and pressure. Adsorption characteristics of CO were determined at CO partial pressure up to 0.39 MPa. Increasing temperature resulted in a decrease in the adsorption amount of CO on Pt electrodes. Under 0.1 MPa of the CO partial pressure, CO almost covers the electrode at 423 K, while a negligible amount is adsorbed at 533 K. At 473 K, CO surface coverage on a Pt electrode in aqueous solution increased with the CO partial pressure.