A new method for the preparation of Pt-Ru and similar bimetallic electrocatalysts is reported. It involves a spontaneous deposition of Pt submonolayers on metallic Ru nanoparticles, which yields electrocatalysts with a considerably lower Pt loading and higher CO tolerance than the commercial Pt-Ru alloy electrocatalysts. The method offers a unique possibility to place the Pt atoms onto the surface of Ru nanoparticles, which very likely makes almost all of them available for hydrogen oxidation, in contrast to the Pt-Ru alloy catalysts that have Pt throughout the nanoparticles. Thus, an ultimate reduction of Pt loading can be achieved. It also facilitates a fine-tuning of the electrocatalyst's activity and selectivity by changing the coverage (the cluster size) of Pt for optimal performance under required CO tolerance levels.