AbstractThe kinetics of the oxygen reduction reaction (ORR) is studied at metal–supporting electrolyte–Nafion three‐phase interfaces. We first demonstrate that the sulfonate anions of Nafion are specifically adsorbed on a wide range of surfaces ranging from Pt(hkl) single‐crystal surfaces, Pt‐poly, Pt‐skin [produced on a Pt3Ni(111) surface by annealing in ultrahigh vacuum, UHV] to high‐surface‐area nanostructured thin‐film (NSTF) catalysts. The surface coverage by sulfonate and the strength of the Pt–sulfonate interaction are strongly dependent on the geometry and the nature of the Pt surface atoms. Also, they are found to behave analogous to (bi)sulfate anion‐specific adsorption on these surfaces, where for the Pt(hkl) surfaces, the trend is Pt(111)>Pt(110)>Pt(100) and for the Pt‐skin surface on Pt3Ni(111), the interaction strength is found to be Pt‐skin<Pt(111). We also found that irrespective of the surface orientation and/or the electronic properties of the surface atoms, the ORR is always inhibited by the presence of ionomers at the electrode surface, confirming that Nafion is not a non‐adsorbing electrolyte. Finally, the knowledge gained from studying well‐defined Pt(hkl) surfaces is applied to propose that deactivation of the ORR on Nafion‐covered high‐surface‐area catalysts is also controlled by specific adsorption of sulfonate anions.