A series of benzonitrile-containing sulfonated poly(phthalazinone ether ketone) ionomers were successfully synthesized via the direct copolymerization of benzonitrile-containing bis(phthalazinone), 3,3'-sulfonated-4,4'-difluorodiphenyl ketone and 4,4'-difluoro-diphenyl ketone. The sulfonation degrees can been readily controlled by changing the feed ratio of monomers. The resulting sulfonated polymers with inherent viscosity ranging from 0.45 to 0.72 dL g(-1) were characterized by H-1 NMR and other technologies. These sulfonated polymers had good solubility in polar aprotic solvents and afforded tough and ductile membranes by casting from DMAc solution at 60 degrees C. Due to the specially designed chemical structures, the membranes showed excellent thermal and oxidative stabilities. Thermogravimetric analysis (TGA) traces demonstrated that all the sulfonated polymers exhibited good thermal stability with initial weight loss > 220 degrees C.The membranes exhibited superior oxidative and hydrolytic stabilities as evidenced by Fenton's reagent. The membranes showed moderate water uptake in low sulfonation degree (<= 1.2), and the proton conductivity of the membrane with sulfonation degree of 1.2 was 1.03 x 10(-3) S cm(-1) at 25 degrees C and 100% relative humidity. The proton conducting and other properties as a membrane can be tailorated by controlling the sulfonation degree. (c) 2007 Elsevier B.V. All rights reserved.