Poly(arylene ether sulfone ketone) (SPESK) multiblock copolymers having highly sulfonated hydrophilic blocks were synthesized and the fuel cell performance with the copolymers was investigated. A membrane electrode assembly (MEA) using an SPESK ionomer with an ion exchange capacity of 1.8 mequiv g(-1) as membrane and Nafion as the electrode binder showed comparable fuel cell performance and ohmic resistance to that using a Nafion NRE 211 membrane at 80 degrees C and 30% relative humidity (RH). A Nafion-free, all-SPESK MEA using SPESK as both the membrane and the binder was operable at 100 degrees C and 50% RH. The fuel cell performance was limited not only by the proton conductivity of the SPESK membrane but also by the low water flux through the membrane and specific adsorption of the ionomer on the platinum catalyst.