Novel sulphonated poly(ether ether ketone) (PEEK) copolymers with controlled degree of sulphonation were synthesized by polycondensation reaction of varying ratios of silylated phenolphthalein, silylated biphenol, 5,5 '-carbonyl-bis(2-fluoro benzene sulphonate) (sulphonated 4,4 '-difluorobenzophenone) and 4,4 '-difluorobenzophenone. The structure of the copolymers was confirmed by using ATR and H-1 NMR techniques. Molecular mass of these samples was determined by GPC. Thermal stability of the samples was evaluated using TGA technique. Methanol diffusion coefficients were in the range of 6.57 x 10(-8) to 2.5 x 10(-6) cm(2)/s. The proton conductivity of the membranes in the temperature range of 25-75 degrees C was found to be 10(-5) to 10(-3) S/cm. (c) 2005 Elsevier B.V. All rights reserved.