The liquidus and solidus temperatures and electrical conductivity of the KF-KCI-KI-K2SiF6 systems were measured as a function of the concentration of KI and K2SiF6. The quasi-binary diagram ([66 mol % KF-34 mol % KCl-75 mol % KI])-K2SiF6 has an eutectic point at 0.23 mol % potassium hexafluorosilicate. Further additions of K2SiF6 result in a liquidus temperature rise due to its low solubility in the system containing potassium iodide. Even small additions of K2SiF6 reduce electrical conductivity significantly.