Bisphenol monomer 4-carboxylphenyl hydroquinone (4C-PH) containing carboxyl groups was synthesized by diazotization reaction of p-aminobenzoic acid and 1,4-benzoquinone and subsequent reductive reaction. Copolymerization of bisphenol A, 4C-PH, sodium 5,5'-carbonylbis(2-fluorobenzene-sulfonate) and 4,4'-difluorobenzophenone at various molar ratios through aromatic nucleophilic substitution reaction resulted in a new sulfonated poly(ether ether ketone) containing pendant carboxyl groups (C-SPEEK). The structures of the monomer 4C-PH and copolymers were confirmed by FT-IR and H-1 NMR. Flexible and transparent membranes with sulfonic and carboxylic acid groups as the proton conducting sites were prepared. The dependence of ion-exchange capacity (IEC), water uptake, proton conductivity and methanol permeability on the degree of sulfonation has been studied. The highest proton conductivity of 0.16 S cm(-1) was obtained from C-SPEEK-4 (IEC = 1.44 mequiv.g(-1)) at 80 degrees C, higher than that of Nafion117 (0.12 S cm(-1)). Incorporating carboxylic acid groups into copolymers led to higher dimensional stability and lower water uptake. The methanol permeability coefficients of C-SPEEK membranes were in the range of 3.54 x 10(-7) cm(2) s(-1) to 5.9 x 10(-9) cm(2) s(-1), Which were significantly lower than that of Nafion117 (1.61 x 10(-6) cm(2) s(-1)). Accordingly, the C-SPEEK membranes exhibited much higher selectivity (conductivity/methanol permeability coefficient) compared with Nafion117 membrane. (C) 2009 Elsevier B.V. All rights reserved.