After treatment in different Fenton's reagents, chemical changes in Nafion® membranes were analyzed by X-ray photoelectron spectroscopy (XPS) and clear evidence of polymer degradation was observed. Exposure of the membrane to 2h of X-ray radiation did not affect the chemical structure of the membrane. However, treatment with various Fenton's reagents indicated that the (CF2) n polymer backbone had decomposed. Fluorine and sulfur XPS peak intensity decreases were consistent with the detection of fluoride and sulfate ions during fuel-cell tests. The increase in oxygen atom concentration suggests that oxygen-rich moieties formed in the membrane. These results indicated that in addition to degradation of the polymer side chain, chemical attack of the CF2 backbone may be the primary reason for extensive fluorine loss and hydrogen crossover in membranes after long-term operation. FTIR spectra showed the formation of CO and S–O–S in the degraded membrane. Two degradation schemes consistent with the results observed have been proposed. Under the same experimental conditions, no detectable changes in XPS spectra were found between a fresh membrane electrode assembly (MEA) and an MEA after long-term operation. These results suggest that degradation occurred mainly within the membrane or at the membrane-electrode interface.