The effects of underpotentially deposited copper (Cu-upd) On the kinetics of the oxygen reduction reaction (ORR) and the hydrogen oxidation reaction (HOR) have been studied at Pt(lll) and Pt(100) surfaces in 0.05 M H2SO4 containing either Cu2+ Or mixtures of Cu2+ and Cl- ions. The results show that Cu-upd has a strong inhibiting effect on both the ORR and the HOR. The deactivation is linked to an enhanced anion adsorption on platinum atoms adjacent to Cu-upd atoms. This mechanism explains the anomalously large inhibition by a very small amount of Cu-upd. The ORR on Cu-upd-modified electrodes proceeds mostly as a 4e(-) reaction pathway with negligible solution phase peroxide formation, ca. 3%. The physical model that appears to realize this result is one in which the active sites for the adsorption of molecular O-2 are a small number of platinum islands which are created when desorption of anions takes place from Cu-upd-unmodified platinum sites. The same sites are also active centers for the HOR.