This work deals with the investigation of the temperatures of primary crystallization and density of the KF + AlF3 + LiF + Al2O3 molten system for three cryolite ratios (CR = 1.3, 1.5 and 1.7). Thermal analysis and the Archimedean method of hydrostatic weighing were used for the determination of studied properties. The dependence of liquidus temperature on composition shows different trends for different CRs, whereas the density increased with increasing content of LiF for all CRs.