A systematic electrochemical study of pseudomorphic Pt(111) thin films is presented. Well-defined Pt-rich surfaces are obtained by inductive heating of bulk PtxRu1-x(111) single crystals (x = 0.1-1) and cooling in an inert gas atmosphere, taking advantage of the segregation behavior of Pt. Due to the difference in atomic diameters of Pt and Ru, compression of the Pt-rich surfaces can be achieved as a function of Pt mole fraction in the bulk alloy single crystal. The electrochemical behavior of characteristic adsorption processes is evaluated by cyclic voltammetry for the samples in contact with 0.1 M HClO4 and 0.1 M H2SO4. Adsorbed hydrogen (H-ad), OHad/O-ad and (bi)sulfate are found to be more weakly adsorbed with decreasing lattice constant of the Pt(111) surface. A volcano-shaped dependence of the ORR activity on lateral compression of the Pt surface could be established in the potential region of low overpotentials (0.95-1.05 V). For 0.1 M HClO4, the highest overall ORR-activity was measured for Pt on Pt0.7Ru0.3(111), exceeding that of pristine Pt(111) distinctly, whereas for 0.1 H2SO4 the highest activity was found for Pt0,9Ru0,1(111). Consequently, the electrochemical and the electrocatalytic behavior is strongly dependent on the electrolyte composition, pointing towards an influence of (bi) sulfate adsorption beyond the site-blocking mechanism commonly referred to. Specific adsorption of spectator species plays a key role for oxygen reduction reaction kinetics. Such correlations between surface electronic structure and reactivity are of major relevance for tailoring the adsorption properties of electro-catalysts, considering both strain and electronic ligand effects.