The surface stress response during the electrooxidation of CO at Pt{111}, Ru{0001}, and Ru-theta=0.37/ Pt{111} textured electrodes was studied in 0 1 M HClO4 electrolytes The surface stress signal resolves for the first time the adsorption of OH- at the CO-covered Ru(0001) surface pnor to significant CO oxidation, a phenomenon that is not discernible in the voltammetry The surface stress signal shows that significant tensile surface stress occurs upon oxidation of the adsorbed CO and occurs at nearly the same potential on Ru{0001} and Ru/Pt{111} surfaces These observations demonstrate that the mechanism of bifunctionality is the OHads provided to the Pt surface sites via Ru sites