Fluorenyl-containing sulfonated poly(arylene ether phosphine oxide)s (sPAEPO) with various sulfonation degrees were synthesized by direct polycondensation of 9,9-bis(4-hydroxyphenyl) fluorene with sulfonated bis(4-fluorophenyl)phenyl phosphine oxide and bis(4-fluorophenyl)phenyl phosphine oxide. The structure of sPAEPO was confirmed by NMR and IR spectroscopy. The transparent, tough, and flexible membranes of sPAEPO were obtained by solution casting. The thermal properties, ion exchange capacity, water uptake, swelling ratio, proton conductivity, and microstructure were investigated. sPAEPO membranes exhibit excellent overall properties. For example, sPAEPO-100 membrane shows a water uptake of 28.6% and a swelling ratio of 10.5% as well as a proton conductivity of 9.4 x 10 (2) S/cm at 80 degrees C. AFM micrographs illustrated that as the sulfonation degree increases, the phase contrast of images and the connectivity of hydrophilic domains increases. For sPAEPO-100, the hydrophilic domains form the connected and narrow channels of an ionic rich phase, which contributes to high proton conductivity as well as dimensional stability. The sPAEPO membranes are expected to be a promising candidate for fuel cell applications. (c) 2008 Elsevier B.V. All rights reserved.