We have studied the effect of nanostructuringin Pt monolayer model electrocatalysts on a Rh(111) single-crystal substrate on the adsorption strength of chemisorbedspecies. In situ high energy resolution fluorescence detectionX-ray absorption spectroscopy at the Pt L3 edge revealscharacteristic changes of the shape and intensity of the “white-line” due to chemisorption of atomic hydrogen (Had) at lowpotentials and oxygen-containing species (O/OHad) at highpotentials. On a uniform, two-dimensional Pt monolayergrown by Pt evaporation in ultrahigh vacuum, we observe asignificant destabilization of both Had and O/OHad due to strain and ligand effects induced by the underlying Rh(111) substrate. When Pt is deposited via a wet-chemical route, by contrast, three-dimensional Pt islands are formed. In this case, strain and Rh ligand effects are balanced with higher local thickness of the Pt islands as well as higher defect density, shifting H and OH adsorption energies back toward pure Pt. Using density functional theory, we calculate O adsorption energies and corresponding local ORR activities for fcc 3-fold hollow sites with various local geometries that are present in the three-dimensional Pt islands.