The dynamic behavior of the interface between a Pt electrode and a Nafion membrane has been demonstrated using in situ Fourier transform infrared (FTIR) spectroscopy and atomic force microscopy (AFM) combined with surface potential measurement (SPoM). The Nafion membrane consists of hydrophobic and hydrophilic domains, both in contact with Pt electrodes. The interface region became more hydrophilic due to an increase of H2O when an electrolyte was dropped on the Nafion membrane. Ion channels on the membrane surface were detected with AFM-SPoM. The size of the ion channels was 40-100 nm. Several tens of channels were observed in a 10x10 mum region. The ratio of two domains for the dry Nafion membrane was smaller than that for the wet Nafion membrane that was expected from the FTIR measurement. From these results, it can be said that an electrochemical reaction site at the interface between the Nafion membrane and the Pt electrode was extremely extended by humidity. These results exhibit a dynamic behavior of interfaces between Pt and ion-exchange polymer. (C) 2003 The Electrochemical Society.