Electrochemical　interfaces　between　polymer　electrolytes　and　electrodes　are　central　to　electrochemical　devices　in　the　global　transition　towards　renewable　energy.　Here　we　show　that　the　adsorption　and　desorption　of　sulfonates　in　Nafion　on　Pt(111)　involve　distinct　elementary　steps,　with　the　latter　proceeding　through　a　coupled　cation-electron　transfer.　Adsorbed　sulfonates　not　only　block　a　fraction　of　surface　Pt　sites　but,　more　importantly,　generate　two　additional　types　of　surface　adsorbate,　OHNafion　and　ONafion,　which　exhibit　distinct　kinetic　properties　from　adsorbed　OH　and　O　on　bare　Pt(111),　respectively.　The　impact　of　the　adsorption　of　sulfonate　groups　in　Nafion　on　the　activity　of　the　oxygen　reduction　reaction　(ORR)　on　Pt　cannot　be　rationalized　by　existing　thermodynamic　descriptors.　The　reduced　ORR　activity　on　the　Nafion-covered　Pt(111)　is　caused　by　the　kinetically　hindered　*O　-＞*OH　conversion　and　*OH　reduction　on　sites　close　to　adsorbed　sulfonates.