- The sound velocity of alloys in the system Bi-Pb has been measured across a wide temperature range to high accuracy using a modified pulse-echo technique. The present work extends the measured temperature range and reduces the uncertainty in the measured values of the sound velocity of Bi-Pb system at high temperatures and in particular of liquid lead bismuth eutectic, which is being considered as a coolant in advanced nuclear systems. The composition range was sampled extensively and measurements were performed for the compositions Bi, Bi 20 Pb 80 , Bi 42 Pb 58 , Bi 48 Pb 52 , Bi 56 Pb 44 (eutectic), Bi 64 Pb 36 , Bi 72 Pb 28 , Bi 86 Pb 14 and Pb, from the liquidus to temperatures of about 1200 K. The temperature dependence of the sound velocity as a function of composition changes markedly in the vicinity of the eutectic composition. These data, together with other experimental inputs are employed to determine the pressure dependence of the interaction between the components in the Bi-Pb system. It was found that the interaction between the liquid solution's components becomes less attractive with increasing pressure. Finally, the interaction parameters obtained were incorporated into a thermodynamic model to study the pressure dependence of Bi-Pb system phase diagram. The position of the eutectic point in the composition-temperature plane is found to shift slowly with increasing pressure.