We report synthesis and properties of a novel series of ammonium-functionalized perfluoroalkylene/aromatic copolymers. In particular, the effect of number and position of ammonium groups on the properties of the copolymer thin membranes was investigated. Fluorenylidene biphenylene groups were used as a scaffold for the ammonium groups. By controlling the chloromethylation reaction conditions, the ammonium groups could be introduced up to 4.0 per fluorenylidene biphenylene unit, resulting in the ion exchange capacity (IEC) of the resulting copolymers ranging from 1.0 to 1.8 meq g(-1). The copolymers provided bendable and transparent membranes by solution casting. The membranes exhibited phase-separated morphology based on the hydrophilic/hydrophobic differences in the copolymer components. A copolymer membrane with IEC = 1.0 meq g(-1) showed high hydroxide ion conductivity (ca. 70mS cm(-1)) at 80 degrees C in water and good alkaline stability in 1M KOH aq. over 1000 h at 60 degrees C. The membrane showed only minor degradation after the long-term alkaline stability test.