Pt electrode/Nafion interface has been characterized in HClO4 aqueous solutions using surface-enhanced infrared absorption spectroscopy (SEIRAS) with the aim of investigating the structure of the interface which plays a crucial role in the performance of polymer electrolyte fuel cells (PEFCs). A potential dependent band was found around 1100 cm(-1) and assigned to the symmetric vibration of the -SO3- groups of the ionomer membrane. Electric field driven orientation/adsorption of the ionomer membrane at the interface was suggested from the potential dependence of the band intensities of -SO3- groups. It was inferred that the -SO3- groups act like counterions at the Pt/ionomer interface to form the electric double layer. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.2137648] All rights reserved.