In this study, a series of poly(styrene-co-vinyl phosphonic acid) [P(S-co-VPA)] copolymers were synthesized by the free-radical copolymerization of styrene and vinyl dimethyl phosphonate followed by alkaline hydrolysis. The P(S-co-VPA) copolymers were characterized by size exclusion chromatography (gel permeation chromatography), Fourier transform infrared vibrational spectroscopy, proton nuclear magnetic resonance, thermogravimetric analysis, differential scanning calorimetry, dynamic mechanical analysis, and electrochemical impedance spectroscopy. Despite the difference between the copolymerization ratios of styrene and vinyl dimethyl phosphonate, the resulting copolymers presented single glass transitions at temperatures that depended on the acidic group amount. The glass transition shifted to a higher temperature and became broader as the amount of phosphonic acid increased. The storage modulus at temperatures higher than the glass transition also increased with increasing acidic groups because of intramolecular and intermolecular interactions. All of the acid copolymers were thermally stable to at least 300 degrees C. A high oxidative stability was found for 3 : 1 P(S-co-VPA), which also presented conductivity values on the order of 10(-6) Omega(-1) cm(-1) at room temperature. The 1 : 1 P(S-co-VPA) membrane presented Arrhenius-type behavior at temperatures from 30 to 80 degrees C and conductivity on the order of 10(-5) Omega(-1) cm(-1). (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 460-471, 2011