A long-term accelerated test (4600 h) of a 25 cm(2) single cell with excess air bleeding (5%) was carried out to investigate the effects of air bleeding on membrane degradation in polymer electrolyte fuel cells. The rate of membrane degradation was negligibly low (fluoride-ion release rate= 1.3 x 10(-10) mol cm(-2) h(-1) in average) up to 2000 h. However, membrane degradation rate was gradually increased after 2000 h. The CO tolerance of the anode gradually dropped, which indicated that the anode catalyst was deteriorated during the test. The results of the rotating ring-disk electrode measurements revealed that deterioration of Pt-Ru/C catalyst by potential cycling greatly enhances H2O2 formation in oxygen reduction reaction in the anode potential range (similar to 0 V). Furthermore, membrane degradation rate of the MEA increased after the anode catalyst was forced to be deteriorated by potential cycling. It was concluded that excess air bleeding deteriorated the anode catalyst, which greatly enhanced H2O2 formation upon air bleeding and resulted in the increased membrane degradation rate after 2000 h. (C) 2007 Elsevier B.V. All rights reserved.