Nafion((R)) is frequently used as electrolyte membrane in polymer electrolyte fuel cells (PEFC). In this Letter the degradation of the Nafion((R)) polymer electrolyte was investigated using X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). XRD showed that the electrode potential and low gas humidification temperature could decrease the crystallinity of Nafion((R)). XPS analysis indicated that the Nafion((R)) was decomposed in the hydrogen potential region of the fuel cell, through the interaction of the hydrophobic (CF2)(n) groups of the membrane with H or/ and C atoms. (C) 2003 Elsevier Science B.V. All rights reserved.