In this work, we investigate the adsorption and desorption processes of sulfonate groups in Nafion ionomer on Pt(111), and demonstrate that interfacial behaviors of anions in the polymer electrolyte drastically depart from those in liquid electrolytes, with important implications for understanding the ORR activity. We first show that the chemical bonding between the sulfonate group in Nafion and the Pt(111) surface is not the sole source of the thermodynamic driving force for its specific adsorption. Then, we provide compelling evidence to show that the adsorption and desorption of sulfonate in Nafion on Pt(111) involve distinct elementary electron transfer steps. Importantly, the presence of adsorbed sulfonate groups not only blocks a fraction of surface Pt sites, but also creates two new type of adsorbates, referred to as OHNafion and ONafion, which possess distinct properties from adsorbed OH and O on bare Pt(111), respectively.