A membrane elect rode assembly (MEA) based on a Nafion 115 membrane shows a 29% loss in power density after continuous testing in a single direct methanol fuel cell for 1000h at 50 degrees C. One of the main reasons for this behaviour is a deterioration of the membrane, namely, its ion-exchange capacity is reduced by 17%. The thermal decomposition behaviour of an untested and degraded membrane is Studied by evolved gas analysis-mass spectrometry, pyrolysis-gas chromatography-mass spectrometry, thermo-gravimetric analysis, differential scanning calorimetry, and in situ FTIR reflectance absorption spectra. The results reveal a decrease in thermal stability of the degraded membrane. The Nafion 115 membrane is suggested to lose ion-exchange capacity after the sulfonic acid group is transformed to a thermally less-stable group, Such as the persulfonic add -SO2OOH group during MEA operation. The Structural changes may prevent the formation of ionic clusters. (C) 2009 Elsevier B.V. All rights reserved.