Data on the density and electrical conductivity of salt melts are of interest for both the assessment of the possibility of their application for the electrolytic production and refining of beryllium and other manufacturing processes and an analysis of the possible interaction of their constituents. Data on the density of molten salt systems comprising beryllium fluoride and alkali-metal chlorides are obtained by hydrostatic weighing. A ball float and a suspension filament are made of platinum. As the crucible and thermocouple case material, beryllium oxide is used. For the BeF2–MCl (M = Li, Na, K, and Cs) and BeF2–(Li–K)eut–Cl systems, 9 to 14 molten salt mixtures containing from 0 to 100% beryllium fluoride are studied when the temperature increases by 100–200 K above the melting temperature of a mixture at an average step of 10 K. Because of the peculiarities of the behavior of individual beryllium fluoride upon heating above the melting temperature (high viscosity and active evaporation), the density of a molten salt is measured by the maximum bubble pressure method. Along with the density, the electrical conductivity of the melts is measured by the capillary method. The measuring cell material is beryllium oxide; as the measuring electrodes, platinum rods 1 mm in diameter are used. The cell constant is determined and regularly controlled using a high-purity potassium chloride melt. All procedures related to the preparation of salt mixtures, their sampling for chemical analysis, and measuring the properties are performed in an isolated dry and additionally purified argon atmosphere. The measurement results are presented in the form of first- and second-order polynomials, which indicate the temperature dependences of density and electrical conductivity for different salt mixture compositions. The values of the simultaneously measured density and electrical conductivity are used to calculate the molar volume and the molar electrical conductivity of the electrolytes. Molar volume isotherms exhibit almost linear behavior, which indicates a weak interaction between the melt constituents. The molar conductivity isotherms are characterized by a typical inflection point, which corresponds to compositions with ~30 mol % beryllium fluoride, which can be related to the formation of complex compounds in the liquid phase.