A series of sulfonated poly(arylene ether nitrile) copolymers containing carboxyl groups were synthesized via a nucleophilic aromatic substitution reaction from phenolphthalein, hydroquinone sulfonic acid potassium salt, and 2,6-difluorobenzonitrile in N-methyl pyrrolidone (NMP) with K2CO3 as a catalyst. The synthesized copolymers had good solubility in common polar organic solvents and could be easily processed into membranes from solutions of dimethyl sulfoxide, NMP, N,N′-dimethyl acetylamide, and dimethylformamide. Typical membranes in acid form were gained, and the chemical structures of these membranes were characterized by Fourier transform infrared analysis. The thermal properties, fluorescence properties, water uptake, ion-exchange capacity, and proton conductivities of these copolymers were also investigated. The results indicate that they had high glass-transition temperatures in the range 151–187°C and good thermal stability, with the 10 wt% loss temperatures ranging from 330 to 351°C under nitrogen. The copolymers showed characteristic unimodal ultraviolet–visible (UV–vis) absorption and fluorescence emission, and the UV–vis absorption, fluorescence excitation, and emission peaks of the copolymers were obvious. Moreover, the copolymer membranes showed good water uptake and proton conductivities at room temperature and 55% relative humidity because of the introduction of both sulfonic acid groups and carboxyl groups into the copolymers, whose contents were in ranges 18.45–67.86 and 3.4 × 10−4 to 3.0 × 10−3 s/cm, respectively. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40213.