Adsorption and desorption of sulfonate groups in Nafion on Pt(111) likely involve distinct electron transfer processes on the molecular level. As the concentration of perchloric acid decreases from 100 to 1 mM, the sulfonate adsorption peak shifts to higher potentials and broadens on the RHE scale , whereas the potential of the desorption peak remains unchanged (Figure ). The contrasting pH dependences indicate that the adsorption and desorption of sulfonate proceed via distinct elementary steps. The lack of shift in the sulfonate desorption peak with pH on the RHE scale could be rationalized by recognizing the electroneutrality requirement in the Nafion phase during electron tranfer processes at the interface . Specifically, we propose that sulfonate desorption proceeds via: