What a polymer electrolyte fuel cell (PEFC) stack consisting of nine cells was operated under low humidification for 2000 h, polymer membrane breakage was found in three of the nine cells. This breakage was found to be caused by chemical deterioration by an OCV accelerated durability test; and the mechanism of deterioration was estimated to be decomposition of the membrane by hydrogen peroxide and hydroxy radicals as by-products of the fuel cell reaction. As a countermeasure, we examined the setting of a radical scavenger layer between the cathode catalyst and the polymer membrane. The effects of seven type of radical scavenger were examined by an OCV accelerated durability test. Cells to which cerium carbonate was added as a radical scavenger, were found to be about 10 times more durable than the standard cell without a radical scavenger. The effectiveness of adding cerium carbonate in an actual-size cell was confined in a daily start and stop (DSS) accelerated durability test base on the most severe practical operating conditions expected.