Abstract <p>The temperature dependences of the ultrasonic velocity in eutectic Pb–Sn melts containing 0, 10, 20, 30, 40, 50, 60, 70, 73.9, 80, 90, and 100 at % Sn have been measured in the range from the liquidus temperature to 1280 K by the pulse-phase method at a frequency of 33.83 MHz. For all compositions, the ultrasonic velocity increases linearly as temperature decreases down to the liquidus temperature. The results obtained for the Pb–Sn system agree well with available literature data. The obtained values of ultrasonic velocity and the literature data on the density of the Pb–Sn melts are used to calculate the adiabatic compressibility and its temperature coefficient. The concentration dependences of the ultrasonic velocity, the adiabatic compressibility, and their temperature coefficients in the Pb–Sn melts have no peculiarities that could indicate a change in the structures of the melts. The adiabatic compressibility and its temperature coefficient of the Pb–Sn melts are close to ideal values. This behavior of the adiabatic compressibility and its temperature coefficient, an insignificant deviation of the molar volume from additivity, and a low enthalpy of mixing show that the Pb–Sn system in the liquid state is close to ideal.</p>