In order to achieve highly conductive polymer electrolytes, room temperature molten salts with high ionic conductivity have been explored, and in situ polymerization of vinyl monomers in the molten salts have been conducted. It is found in this study that 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) and 1-butylpyridinium tetrafluoroborate (BPBF4) form room temperature molten salts, and these molten salts exhibit high ionic conductivities of 2 x 10(-2) and 3 x 10(-3) S cm(-1) at 30 degrees C, respectively. Certain vinyl monomers can be polymerized in the molten salts by radical polymerization. In situ polymerization of suitable vinyl monomers gives transparent, mechanically strong and highly conductive polymer electrolyte films. For example, 2-hydroxyethyl methacrylate network polymers in which BPBF4 is dissolved exhibit an ionic conductivity of 10(-3) S cm(-1) at 30 degrees C. (C) 2000 Elsevier Science Ltd. All rights reserved.