The enthalpy of formation of ternary Pb-Se-Sn alloys was measured at 1036, 1118, and 1243 K by direct reaction calorimetry (drop method) using a high-temperature Calvet calorimeter. The results are given as iso-enthalpy curves with respect to composition. Some phase boundaries were also deduced. From the extrapolation of the results obtained for ternary alloys we could deduce the enthalpy of formation of Se-rich Pb-Se and Sn-Se liquid binary alloys not known yet.