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 O-2 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 contribute to the observed activity. The results illustrate that placing a Pt monolayer on a suitable metal nanoparticle substrate is an attractive way of designing better O-2 reduction electrocatalysts. Also, by using this method the Pt content is reduced to very low levels. The Pt mass-specific activity of the Pt/Pd/C electrode is 5-8 times higher than that of the Pt/C electrocatalyst. The noble metal (Pt + Pd) mass-specific activity is two times higher than that of Pt/C.