This paper aims at the comparison of two differently operated Celtec((R))-P 1000 MEAs: one MEA is operated in a start/stop cycling mode (12 h operation at 160 degrees C followed by 12h shutdown), the other MEA is continuously operated at 160 degrees C. We demonstrate more than 6000h total operation time with more than 240 start/stop cycles, while the test is still ongoing. The degradation rate is 0.2 mV cycle(-1) or 11 mu V h(-1) on a time basis. The continuously operated MEA is operated more than 6000 h with a degradation rate of approximately 5 RV h(-1). Through separation of the individual cathodic loss terms, a detailed insight into the doubled time-based degradation rate under start/stop cycling conditions could be given. Both Ohmic resistances and oxygen reduction overpotentials are basically identical in MEAs under continuous and start/stop cycling operation. In the start/stop cycled MEA, however, significantly increased cathodic mass transport overpotentials are observed as a result of enhanced corrosion of the cathode catalyst support, which confirms the generally discussed reverse-current mechanism under start/stop cycling conditions. Results from a newly developed MEA demonstrate that this mechanism can successfully be mitigated through improvement of the materials used in the MEA. (c) 2007 Elsevier B.V. All rights reserved.