In the present work, we focus attention on the effect of LiI addition to newly discovered pure lithium ion conductor, LiBH4. Solid solution of the composition LiBH4-LiI (LiI: 6.25-33.3 mol%) was synthesized by solid state reaction. Electrical conductivity was measured from room temperature to 140 degrees C by ac impedance method, which revealed the fast-ion conduction phase of LiBH4 can be stabilized to lower temperature, below the room temperature. Solid solution with LiI showed higher conductivities and lower activation energies in comparison with LiBH4. Powder XRD measurement was carried out at 120 degrees C (just above the transition temperature of LiBH4). The lattice constants of the solid solution were determined. DSC measurement showed a systematic compositional dependence on the transition temperature. The stabilization mechanism was discussed. (C) 2010 Elsevier B.V. All rights reserved.