Specific adsorption of the sulfonate groups in Nafion on Pt(111) is well established, however, the origin of its unique irreversible cyclic voltammetric (CV) features remains unclear. CV of Pt(111) in 0.1 M HClO4 shows the well-established features in the hydrogen underpotential deposition/oxidation (Hupd), double layer, and hydroxide adsorption/desorption regions in the potential ranges of 0.07-0.35 , 0.35-0.55, and 0.55-0.85 V, respectively (Figure , black trace), which are consistent with the literature and demonstrate the reliability of our single crystal voltammetries. All potentials in this work are referenced to the reversible hydrogen electrode (RHE) scale unless noted otherwise. When the Pt(111) surface is coated with a layer of Nafion ionomer by dropcasting and drying (see Methods for details), two additional sets of features appear in the CV (Figure , red trace). The set of redox features in the double layer region were assigned to the specific adsorption and desorption of sulfonate group on Pt(111) , whereas the set of CV redox peaks in the hydroxide region, i.e., 0.55-0.95 V, have not been extensively discussed in the literature. Intriguingly, the sulfonate group in methylsulfonic acid (MSA) does not adsorb specifically on Pt(111), as evidenced by the lack of CV peaks in the double layer region (Figure ), which provides evidence for the lack of specific adsorption of anions other than OH -. Consistently, no additional peak was present in the CV when 0.1 mM sodium butylsulfonate was added to 0.1 M HClO4, suggesting the lack of specific adsorption of the sulfonate group (Figure ). In contrast, characteristic adsorption peaks were observed in the CV when 0.1 mM sulfate was introduced to 0.1 M HClO4 (Figure ), which is consistent with the literature . No comparable CV peak in the double layer region to that of Nafion was observed with perfluorobutanesulfonic acid, octanesulfonate and dodecanesulfonate (Figures S4-S6, Table ), further supporting the lack of specific adsorption of alkyl sulfonates. Likely reasons for the suppression of CV features in the Hupd and hydroxide regions in alkyl sulfonic acids, including the possibility of impurities in the electrolyte, are discussed in the Supplementary Note 1. The adsorption and desorption of the sulfonate groups in Nafion is unlikely impacted by the redox processes occurring at more positive potentials as the associated peaks remain unchanged when the upper bound potential of the CV is shifted from 0.95 to 0.60 V (Figure ). Results discussed above highlight a mystery, i.e., sulfonate group does not specifically adsorb on Pt(111) when attached to an alkyl chain but readily adsorbs when it is in the Nafion ionomer. These results suggest that caution must be exercised when transferring electrochemical insights gained from solid/liquid interfaces to solid/polymer interfaces.