Branched sulfonated poly(ether ketone sulfone)s (Br-SPEKS) were prepared with bisphenol A, bis(4-fluorophenyl)sulfone, 3,3'-disodiumsulfonyl-4,4'-difluorobenzophenone, and THPE (1,1,1-tris-p-hydroxyphenylethane), respectively, at 180 degrees C using potassium carbonate in NMP (N-methylpyrrolidinone). THPE, as a branching agent, was used with 0.4 mol % of bisphenol A, to synthesize branched copolymers. Copolymers containing 10-50 mol % disulfonated units were cast from dimethylsulfoxide solutions to form films. Linear sulfonated poly(ether ketone sulfone)s (SPEKS) were also synthesized without THPE. The films were converted from the salt to acid forms with dilute hydrochloric acid. A series of copolymers were studied by Fourier transform infrared, H-1-NMR spectroscopy, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Sorption experiments were conducted to observe the interaction of sulfonated polymers with water and methanol. The ion-exchange capacity (IEC), a measure of proton conductivity, was evaluated. The synthesized Br-SPEKS and SPEKS membranes exhibit conductivities (25 degrees C) from 1.04 x 10(-3) to 4.32 x 10(-3) S/cm, water swell from 20.18 to 62.35%, IEC from 0.24 to 0.83 mequiv/g, and methanol diffusion coefficients from 3.2 x 10(-7) to 4.7 x 10(-7) cm(2)/S at 25 degrees C. (c) 2008 Wiley Periodicals, Inc.