N-vinylimidazolium tetrafluoroborate (VyImBF(4)) was prepared by the neutralization and polymerized to obtain a model of molten salt polymers. After polymerization, obtained P(VyImBF(4)) showed much lower ionic conductivity (2.0 x 10(-9) S cm(-1) at 30 degrees C) than the corresponding monomeric VyImBF(4) (1.0 x 10(-4) S cm(-1) at 30 degrees C). The conductivity increased with the increase of lithium salts (LiTFSI and LiBF4) concentration up to 25 mol% to Im(+). unit. However, the ionic conductivity was decreased once at the concentration of 50 mol%. Then the conductivity increased again about one order at 60 mol% addition and reached a constant value for highly concentrated samples. All the samples were turbid when approximately more than 60 mol% salt was added, suggesting phase separation. The microscopic phase separation was effective to keep the ionic conductivity high in P(VyImBF(4)). When LiCl was added to P(VyImBF(4)), the maximum conductivity was found at 50 mol%. P(VyImBF(4))/LiCl mixture showed lower ionic conductivity than other mixtures did for any salt concentration. The salt having low lattice energy led to high ionic conductivity of the polymer salt. (C) 2000 Elsevier Science Ltd. All rights reserved.