The durability of a membrane-electrode assembly based on a sulfonated poly(arylene ether sulfone ketone) (SPESK) multiblock copolymer membrane having highly sulfonated hydrophilic blocks was evaluated at various cell currents in a polymer electrolyte fuel cell. The electrochemical properties and drain water collected from the anode and cathode were monitored during the test, followed by post-test analyses of the membrane. With a lower current test, the percentage remaining of the original weight-averaged molecular weight (M-w)(orig) of the SPESK membranes after the test decreased, and the sulfate ion release rate increased. In addition, the mass activity, evaluated from the current-potential (I-E) curves and oxide formation charge current (Q(oxide)) determined from cyclic voltammograms on the cathode were found to decrease with the lower current test. Fluoride ions originating from the catalyst layer were also detected in the drain water. There were greater amounts of fluoride ion with the lower current tests, similar to the case for the sulfate anion. Thus, it was concluded that the electrolyte binder in the catalyst layer was degraded. The effect of low current density may be associated with high cathode potential. (C) 2011 Elsevier Ltd. All rights reserved.