CO adsorption and stripping on Pt(111) electrodes modified by irreversibly adsorbed selenium and tellurium has been studied in sulfuric acid medium. Selenium and tellurium do not exhibit any catalytic effect on the CO oxidation process. Neither adatom can act as a bifunctional catalyst since both are in their reduced form at the onset of CO oxidation and therefore cannot transfer the oxygen necessary to CO oxidation. FTIR spectra show that selenium simply behaves as a third body with respect to CO adsorption and oxidation, whereas tellurium exhibits an electronic effect that stabilizes adsorbed CO on the Pt(111) electrode. Electrochemical and FTIR measurements also demonstrate that linear CO is the only species on the electrode surface at high adatom coverage. For coadsorbed CO and selenium a mixed adlayer is proposed.