The electrochemical adsorption of underpotential deposited hydrogen (upd-Had) and OHad on structurally well-defined PtxRu1-x/Ru(0001) surface alloys was investigated by cyclic voltammetry and density functional theory (DFT) calculations. The adsorption energies of both upd-Had and OHad decrease with increasing Pt content in the adsorption ensemble, shifting the onset of upd-Had and OHad formation to increasingly cathodic and anodic potentials, respectively. For bare Ru(0001) and for Ru3 sites in the surface alloy, the stability regions of these two species overlap or almost overlap, respectively. Similar to previous findings for upd-Had adsorption/desorption on partly Pt monolayer island covered Ru(0001) surfaces (J. Phys. Chem. B 2004, 108, 14780), we find a sharp peak at ∼100 mV vs. RHE in each scan direction, which is attributed to a Pt catalyzed OHad↔ upd-Had replacement on Ru3 sites, via adsorption on Pt rich sites and spill-over to Ru3 sites. The decrease of the integrated charge in these peaks with the third power of the Ru surface concentration, which for a random distribution of surface atoms reflects the availability of Ru3 sites, supports the above assignment.