An experimental thermodynamic study has been performed on some liquid alloys of the ternary system Ag-Bi-Sn by making use of the torsion-effusion method. Through the measurement of the vapor pressure of Bi in thermodynamic equilibrium with the liquid phase, the Bi activity as a function of Bi atomic fraction, X-Bi, was obtained. Three ternary alloys having a fixed ratio rho = X-Sn/X-Ag (rho: 0.095; 0.200; 0.333) were examined starting from the original alloy composition and changing X-Bi by selected short isothermal vaporizations of Bi. In this way, it was found that all the alloy tested had positive deviations from ideality. Some thermodynamic quantities were evaluated at T = 1000 K. They are Bi mixing partial molar quantities such as (Delta(mix)G) over bar (T)(Bi), (Delta H-mix) over bar (T)(Bi), (Delta S-mix) over bar (T)(Bi) and mixing integral molar quantities, i.e., Delta(mix)G(T)(xs) and Delta(mix)G(T)(re), which are excess and real, respectively.