Compared to Pt/C, the HOR activity of Pt-Ru alloys in alkaline electrolyte is exceptionally high. Nevertheless, it remains unknown whether this enhancement is due to a bifunctional mechanism involving Pt and Ru as active sites or an electronic effect of Ru on Pt. In this study, we distinguish between those fundamental differences using Ru@Pt core-shell nanoparticles as a model system. Ru@Pt catalysts were prepared from submonolayer to multilayer Pt coverage. The exposure of Ru on the surface of the catalyst was analyzed by cyclic voltammetry, showing that Ru is solely exposed on the surface of Ru@Pt particles with low Pt-coverage. The thickness of the Pt-shell was characterized by CO stripping in H2SO4, allowing to distinguish between single and bilayered Ru@Pt catalysts. Determining the HOR/HER activity of these catalysts in 0.1 M NaOH revealed that fully Pt-covered Ru is more active than partially covered Ru@Pt nanoparticles. Hence, the participation of Ru as active site in a bifunctional mechanism is of minor importance with respect to the HOR/HER activity compared to its influence on the electronic structure of Pt. Similar to Pt-Ru alloys, the most active Ru@Pt core-shell nanoparticles show a 4 to 5-fold enhancement of the surface-normalized HOR/HER activity compared to Pt/C. (C) The Author(s) 2018. Published by ECS.