CO adsorption and stripping on Pt(111) electrodes modified by irreversibly adsorbed bismuth and arsenic were studied in sulphuric acid medium. Coadsorbed bismuth and CO form a mixed adlayer, whereas arsenic tends to desorb in the presence of CO. Both adatoms modify the CO stripping process, resulting in the catalysis of CO oxidation, in which arsenic is more effective than bismuth. Fourier transform IR (FTIR) studies demonstrate that bismuth stabilizes the adsorbed CO, while arsenic shows the opposite effect. In both cases, linear CO is the only species present on the electrode surface at high adatom coverage. The catalysis mechanism is an adatom-mediated oxygen transfer for both cases, with an additional electronic effect when arsenic is on the surface. Using the CO stripping charge after adequate correction, the values of the CO coverage for the different adatom coverages were calculated. The results of the CO coverage agree well with the behaviour shown in the FTIR experiments.