2) The peak shifts are almost identical for both OHnormal and OHNafion when solvent is switched from H2O to D2O (Figure ), which is consistent with both bands correspond with hydroxyl species. 3) In addition to the adsorption/desorption features of OH, the presence of coadsorbed sulfonate also impacts the CV feature attributed to the oxidation of adsorbed hydroxyls (OHad) to adsorbed oxygen atoms (Oad). The peak at 1.065 V in the anodic scan on a Nafion-free Pt(111) in perchloric acid has been assigned to the OHad-to-Oad conversion (Figure ) . It follows that if a different type of adsorbed hydroxyl species is present in a Nafion-covered surface, a corresponding CV oxidation feature is expected, which is indeed the case. On Nafion-covered Pt(111), a new peak at 1.115 V attributable to adsorbed O on Pt sites influenced by adjacent adsorbed sulfonate, denoted as ONafion, was observed, and its intensity increases in sync with the sulfonate and OHNafion features (Figure ). The CV peak of OHNafion-to-ONafion also shifts 36 mV when switching the solvent from H2O to D2O (Figure ), which is consistent with the scission of the O-H(D) bond in the conversion. AIMD simulations suggested that the distinct microenvironment of OHNafion from