<p>The density and thermal expansion of a liquid rubidium–lead alloy containing 50 at. % Pb were investigated for the first time by the gamma-ray attenuation technique in the temperature range from the liquidus to ∼1000 K. It was found that the molar volume of the Rb₅₀Pb₅₀ melt is 30 % less than the molar volume of an ideal solution of the same composition. The temperature dependence of the melt density is highly nonlinear, therefore the volumetric coefficient of thermal expansion decreases by more than 1.5 times with an increase in temperature from 864 to 1010 K. Based on modern concepts of the structure of liquid metal systems with partially ionic nature of interatomic interaction, the behavior patterns of the volumetric properties of the liquid alloy are briefly analyzed.</p>