The kinetics of oxygen reduction was studied in acid solutions on Pt monolayers deposited on a Pd(111) surface and on carbon-supported Pd nanoparticles using the rotating disk-ring electrode technique. These electrocatalysts were prepared by a new method for depositing Pt monolayers involving the galvanic displacement by Pt of an underpotentially deposited Cu monolayer on a Pd substrate and characterized by scanning tunneling and transmission electron microscopies. The kinetics of O2 reduction shows a significant enhancement at Pt monolayers on Pd(111) and Pd nanoparticle surfaces in comparison with the reaction on Pt(111) and Pt nanoparticles. The four-electron reduction, with a first-charge transfer-rate determining step, is operative on both surfaces. The observed increase in the catalytic activity of Pt monolayer surfaces compared with Pt bulk and nanoparticle electrodes may reflect decreased formation of PtOH. An enhanced atomic scale surface roughness and low coordination of some atoms may contribut...