Novel multi-component molten salt systems containing iodides, LiF–LiBr–LiI, LiF–NaBr–LiI, and LiF–LiCl–LiBr–LiI, were investigated for use as electrolytes in high temperature molten salt batteries to improve the discharge rate-capability. The iodide-based molten salts showed higher ionic conductivity (∼3Scm−1 at 500°C) than conventional LiCl–KCl, and had low enough melting points (below 400°C) that can be used in practical high temperature molten salt batteries. The iodide-based salts showed instability at temperatures higher than 280°C in dried air. The decomposition mechanism of iodide-based molten salts was discussed, and it was found that elimination of oxygen from the environment is effective to stabilize the iodide-based molten salts at high temperatures.