Electrochemical and nonelectrochemical reactions of the H2O2/CO system on a Pt electrode were studied in 2 M H2SO4 with cyclic voltammetry and on-line mass spectrometry. It was found that a complete CO monolayer is chemically stable in the presence of dissolved H2O2. However, H2O2 oxidizes adsorbed CO to CO2 at lower potentials than those required for electrochemical CO adsorbate oxidation when the Pt electrode is partially covered by CO. Furthermore, it was observed that a complete CO monolayer on Pt inhibits the electrochemical reduction of H2O2 to H2O in CO-containing electrolyte.