The synthesis of novel soluble copolyarylenes, their functionalization with sulfonic and carboxylic acid groups, and the determination of fuel cell relevant parameters (swelling behavior, methanol permeation, and ionic conductivity) are described. The Ni(O)-catalyzed homocoupling reaction of aryl chlorides was employed for the polymerizations. Carboxylic acid groups were incorporated by copolymerization of methyl 2,5-dichlorobenzoate and subsequent hydrolysis. The composition was varied from 53 to 100 mol % carboxylic acid groups. Sulfonic acid groups were introduced by sulfonation with chlorosulfonic acid. Flexible and transparent membranes with sulfonic and/or carboxylic acid groups were prepared that exhibited higher proton conductivities (values in the range of sigma = 0.11-0.23 S/cm) compared to those of Nafion and sulfonated PEEK as a result of higher ion exchange capacity and water content. Incorporation of carboxylic acid groups led to a reduced water uptake but lower conductivities.