A series of anion exchange membranes of poly (fluorenylene ether sulfone) containing tertiary sulfonium hydroxide-functionalized fluorenyl groups were synthesized by sequential polycondensation, chloromethylation, substitution with dimethyl sulfide and ion exchange. They showed excellent solubility in polar aprotic solvents. Consequently, flexible and tough alkaline membranes with varying ionic contents were obtained by an anion exchange of tertiary sulfonium chloride polymers with 1.0 M KOH at room temperature. Different levels of substitution were performed to achieve high ionic conductivity as well as upholding the membranes' mechanical stability. The tertiary sulfonium membranes demonstrated lower water uptake compared to quaternary ammonium membrane. High hydroxide ion conductivity was achieved up to 18.3 mS cm(-1) at 80 degrees C with the membrane of the highest ion exchange capacity (IEC, 1.51 mmol g(-1)). The resulting alkaline polymers were characterized by 1H NMR, FT-IR, thermogravimetric analysis (TGA), water uptake, IEC, atomic force microscopic (AFM) images. (C) 2016 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.