Copolymers of vinylphosphonic acid (VPA) and 4-vinylimidazole (4-VIm) were synthesized with various monomer feed ratios by free-radical polymerization. The copolymers were characterized by H-1-NMR, and the copolymer composition was determined via elemental analysis. The protonation of the heterocycles at the 'free' nitrogen was confirmed by FT-IR. Thermogravimetric analysis (TG) illustrates that the copolymers are stable up to 200 degreesC. The morphology of the copolymers was characterized by wide-angle X-ray scattering (WAXS). For all the copolymers, no distinct glass transition temperatures were found up to the onset of degradation. The temperature dependence of the alternating current (AC) conductivities of the samples were investigated by means of impedance spectroscopy. The direct current (DC) conductivities of the samples were derived from the AC conductivity data. (C) 2003 Elsevier B.V. All rights reserved.