The activity of PbTe in liquid Te-saturated PbSe-PbTe solid solutions in the temperature range from 730 to 830 K was measured by an electrochemical technique using LiCl-KCl+5 wt.% PbCl2 as the molten salt electrolyte. The activity of PbSe was calculated by Gibbs-Duhem integration using the alpha-function. From the e.m.f. values of the cell measured at different temperatures, the partial, integral and excess molar thermodynamic quantities were calculated. All the excess partial molar thermodynamic quantities at infinite dilution were accurately estimated by Chiotti’s method. Continuous variation of activity and of the activity coefficient of both the components with composition, indicates that the PbSe-PbTe system is completely miscible and consists of a single-phase field throughout the entire range of composition in the range 730-830 K.