Novel electrocatalysts based on Pt submonolayer deposits on Ru and Au nanoparticles, and on a Au(1 1 1) surface, have been prepared by two new methods for noble metal monolayer deposition. These were tested for H-2 and H-2/CO oxidation and O-2 reduction kinetics. Our recently reported methods for controlled submonolayer-to-multilayer deposition of Pt involve spontaneous deposition of Pt on Ru and Pt deposition by redox replacement of a Cu upd monolayer on Au. The Pt mass-specific activity for H-2 oxidation of the PtRu20 electrocatalyst was found to be three to four times higher than that of commercial catalysts at 25 degreesC. The CO tolerance appears to be also higher than those of commercial catalysts under conditions of rotating disk experiments. Fuel cell tests at 80 degreesC have shown practically the same activity of the PtRu20 electrocatalyst for H-2 oxidation as that of the catalyst containing a 10 x larger Pt loading. Extended X-ray absorption fine structure spectroscopy measurements showed that Pt is coordinated with three to four Ru atoms in this catalyst, with a bond length of 2.68 A between Pt and Ru as in PtRu alloys. A Pt0.75Pd0.25 monolayer on Au/C is a very active electrocatalyst for O-2 reduction that equals the activity of a Pt/C catalyst with a 2.5 times larger Pt loading. The activity of a Pt monolayer on Au(1 1 1) is also considerable. A single Tafel slope of -110 mV for the Pt0.75SPd0.25/Au/C catalyst, and a -120 mV for Au(1 1 1), may indicate a smaller adsorption of OH on Pt and Pt0.75Pd0.25 on Au in comparison with bulk or nano Pt. Further work utilizing a Pt monolayer on metal nanoparticles seems to be a promising approach for both reduction of noble metal loading and designing electrocatalysts with improved properties. (C) 2003 Elsevier Ltd. All rights reserved.