The KBF4 impact on the physical-chemical properties of the KF-AlF3 and KF-NaF(10 wt%)-AlF3 melts with cryolite ratio (CR) 1.3-1.5 was studied at temperatures 700-800 degrees C. The KBF4 additions (up to 15 mol%) to the potassium-cryolite-based electrolytes increase the liquidus temperature, however, the KBF4 presence in the cryolite-alumina melt improves the Al2O3 solubility and enlarges the range of homogeneity in the (cryolite-KBF4)-Al2O3 system. The KBF4 additions (within a concentration range of 0-10 mol%) do not noticeably impact the electrical conductivity of the KF-AlF3-KBF4 melt (CR = 1.3). The mass loss of the KF-AlF3-KBF4 samples with low CR determined by thermogravimetry analysis over two hours is not higher than 3% at temperature 800 degrees C. (C) 2015 The Electrochemical Society. All rights reserved.