Sulfonated poly(ether ether ketone sulfone) (SPEEKS) and sulfonated poly(ether ether ketone ketone sulfone) (SPEEKKS) copolymers with different degree of sulfonation (DS) were synthesized by aromatic nucleophilic polycondensation of disodium 3,3'-disulfonate-4,4'dichloro-diphenylsulfone (SDCDPS), tertbutylhydroquione, and 4,4'-difluorobenzophenone or 1,4'-bi(4-fluorobenzoyl) benzene. Prepared sulfonated copolymers were characterized by Fourier transform infrared spectra, thermogravimetric analysis, and differential scanning calorimetry. The transmission electron microscope was used to investigate the microstructure of membranes. The different distance between two adjacent sulfonic groups in two series of membranes resulted in different physical and electrochemical properties between two kinds of membranes with the same DS. The proton conductivity, ionic exchange capacity and water uptake of SPEEKS membranes were higher than those of SPEEKKS membranes while the mechanical strength of SPEEKS membranes was lower than that of SPEEKKS membranes at the same DS. Moreover, the SPEEKKS membranes with DS equals to 0.8 showed a good combination of a high proton conductivity (0.046 S/cm at 25 degrees C, 0.061 S/cm at 80 degrees C), acceptable water uptake (33-65 wt %), excellent mechanical strength (tensile strength reached 49.7 MPa), and good thermal properties (T-g above 250 degrees C, T-d5%, above 300 degrees C). It suggested that this could be a promising membrane for proton exchange membrane fuel cell application. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 858-866, 2009