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To obtain clues to how to improve the effectiveness of cathode nanocatalysts in fuel cells, the interfacial structure of Nafion−Pt/C and Nafion−Pt 3 Co/C in fully humidified N 2 at room temperature was, for the first time, investigated by in situ attenuated total reflection Fouriertransform infrared (ATR-FTIR) spectroscopy by use of a membrane-electrode assembly (MEA) type cell without any supporting electrolyte solution. We identified four types of adsorbed species at the interface with changing the electrode potential, that is, sulfonate anions (SO 3 − ) with ether groups (C−O−C) in the Nafion side chain, water molecules (adsorbed H 2 O ad and/or hydrating SO 3 − groups), and hydrated protons [H + (H 2 O) n ]. For a Pt skin layer formed on Pt 3 Co/C, it was clearly observed that the band intensities assigned to the water bending mode δ(HOH) of both H + (H 2 O) n (1800 and 1720 cm −1 ) and H 2 O ad (1610 cm −1 ) decreased with increasing potential more positive than 0.2 V vs RHE, accompanied by increases in those of ν s (SO 3 − ) (1050 cm −1 ) and ν(C−O−C) (965 cm −1 ).This indicates that the SO 3 − groups with an ether side chain in Nafion adsorbed specifically on the catalyst surface, while expelling H + (H 2 O) n and H 2 O ad originally adsorbed at less positive potentials. We also found that the onset potential for the specific adsorption of the SO 3 − group in the Nafion side chain on the Pt skin layer formed on Pt 3 Co was less positive by ca. 0.2 V than that on Pt, which is a tendency similar to that found for the specific adsorption of ClO 4 − anions on these electrodes in our recent results obtained by use of the electrochemical quartz crystal microbalance (EQCM).