A series of Sulfonated Poly (fluorenyl ether ketone) ionomers containing aliphatic functional segments were synthesized and characterized. The monomer 4,4'-Dihydroxy-alpha, omega-diphenoxydecane with aliphatic group was conveniently prepared from hydroquinone and 1,10-dibromodecane. A series of sulfonated aliphatic functional groups containing poly(fluorenyl ether ketone)s with different aliphatic group content were successfully synthesized and characterized in detail, in particular with respect to properties relevant for their application as membrane materials in proton exchange membrane (PEM) fuel cells. Tough and transparent membranes were formed by casting from their solutions. The effects of alkyl groups were investigated by comparison of the PEM properties of the copolymers with different content of aliphatic component on the copolymer chain. The introduction of aliphatic segments can provide an enhanced water uptake, increased proton conductivities, but worse oxidative stability. Transmission electron microscope (TEM) images of j and f revealed an improved phase separation structure under the effect of aliphatic groups. The as-made membranes can also exhibit comparable or much better single cell performance than Nafion (R) 117 at 75 degrees C-95 degrees C under full or partial relative humidification. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.