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 °C. The KBF4 additions (up to 15 mol.%) to the potassium-cryolite-based electrolytes increase the liquidus temperature and the alumina solubility, the latter provides enlarging the homogeneity range of the (cryolite-KBF4)-Al2O3 system. The boron-containing cryolites have density lower than that of boron-free electrolytes. The electrical conductivity of the KF-AlF3-KBF4 mixtures (CR=1.3) within the KBF4 concentration range 0-10 mol.% is practically constant.