The use of a hydrogen purge for startup and shutdown (H-2-SU/SD) process of polymer electrolyte fuel cells has been proposed, which suppresses the generation of internal current during the SU/SD, process so-called "reverse current", and the severe carbon oxidation reaction (COR) in the cathode. However it was found that the COR was still caused during this H-2-SU/SD process, even though it was less severe than that during the usual SU/SD process, i.e., the anode gas was successively cycled between air and H-2. In order to clarify the mechanisms of the COR, we investigated (1) the effect of the presence of Pt catalyst, (2) the timing, and (3) the effect of Pt oxidation state. These results indicated that the COR was accelerated by the Pt catalyst in the cathode and was decelerated with increasing cathode potential during the H-2-SU/SD process. We propose that the COR is caused by a shortage of protons associated with both the reduction of the Pt oxide and the oxygen reduction reaction at the reduced Pt. (C) The Author(s) 2017. Published by ECS. All rights reserved.