Optical and electrical properties of the LiBO2–LiF composite thin films prepared by thermal evaporation have been studied for their use as ion conductors in solid‐state electrochromic (EC) systems. All the LiBO2–LiF films exhibited a high degree of optical transmission (over 90%) and the best lithium ion conductivity attainable was about 8×10−7S cm−1, a value considerably higher than that for the ionic conductivity of LiBO2. A typical activation energy of 0.545 eV was found for the composite film. The films obtained thus have a good potential for use as ion conductors in EC devices. ©1995 American Institute of Physics.