A series of sulfonated poly(aryl ether ketone)s (SPAEKs) were prepared by aromatic nucleophilic polycondensation of 2,6-dihydroxynaphthalene with 5,5'-carbonyl-bis(2-fluorobenzenesulfonate) and 4,4'-difluorobenzophenone. The structure and degree of sulfonation (DS) of the SPAEKs were characterized using H-1 NMR spectroscopy. The experimentally observed DS values were close to the expected values derived from the starting material ratios. The thermal stabilities of the SPAEKs were characterized by thermogravimetric analysis, which showed that in acid and sodium salt forms they were thermally stable in air up to about 240 and 380 degreesC, respectively. Transparent membranes cast from the directly polymerized SPAEKs exhibited good mechanical properties in both dry and hydrated states. The dependence of water uptake and of membrane swelling on the DS at different temperatures was studied. SPAEK membranes with a DS from 0.72 to 1.60 maintained adequate mechanical properties after immersion in water at 80 degreesC for 24 h. The proton conductivity of SPAEK membranes with different degrees of sulfonation was measured as a function of temperature. The proton conductivity of the SPAEK films increased with increased DS, and the highest room temperature conductivity (4.2 x 10(-2) S/cm) was recorded for a SPAEK membrane with a DS of 1.60, which further increased to 1.1 x 10(-1) S/cm at 80 degreesC. (C) 2004 Wiley Periodicals, Inc.