The influence of underpotentially deposited Cu adlayers on the electrocatalytic reduction of oxygen at Pt(lll) has been studied in 0.05 M H2SO4 solutions using hanging meniscus rotating-disk (HMRD) voltammetry and electrochemical scanning tunneling microscopy (STM). Oxygen reduction at clean bare Pt(lll) proceeds by a direct four-electron transfer with the formation of H2O as the primary reaction product. After the formation of the first underpotentially deposited Cu adlayer with (root 3 X root 3)R30 degrees structure, the oxygen reduction current decreases to a steady-state value which is almost half that observed at bare Pt. This partial inhibition provided by the Cu adatoms can be explained by a change in the oxygen adsorption mechanism from the bridged orientation, favoring a four-electron transfer, to the end on orientation, favoring a two-electron transfer. The effect of coadsorbed halides on underpotential deposition (UPD) as well as the oxygen reduction reaction, has also been examined. Oxygen reduction at Cu-modified Pt(lll) in the presence of chloride was completely inhibited after the first UPD process. Further, oxygen reduction in a pure H,SO, solution on bare Pt(lll) was carefully studied using HMRD voltammetry. The oxygen reduction current at 0.02 V was almost half the constant limiting current observed in the potential region between 0.5 and 0.3 V. This result can also be explained by a change in the oxygen adsorption mechanism from the bridged to the end-on orientation.