The performance of Pt-based oxygen reduction reaction (ORR) catalysts is governed not only by adsorption energetics but also by the thermodynamic accessibility of near-surface structures and the resulting adsorbate coverage. Here, we combine a density-functional-theory-trained cluster expansion model with semi-grand-canonical and kinetic Monte Carlo simulations to resolve the structure–activity relationship on Pt–Cu(111) surfaces for ORR. Equilibrium sampling within the bulk Pt 3 Cu chemical pot