Physical-chemical properties (liquidus temperature, density, alumina solubility) in the KF - AlF3 melts with [KF]/[A1F3] = 1.3 containing the KBF4 and B203 additives have been studied. Compositions of molten salts with the effective protecting and refining functions, low density, which can be successfully used as fluxes during metalothermal production of Al-В alloys, have been determined.