The thermodynamic properties of ternary Al-Cu-Zn alloys containing 25 to 62 at % Al have been determined by electromotive force (EMF) measurements between 420 and 920-degrees-C by an aluminum concentration cell of the type Al[AlCl3-LiCl-KCl] alloy. This concentration cell was evaluated via comparison with known partial free energy for Al measured in liquid Al-Cu alloys by Wilder, and several liquidus temperatures of Al-Cu phase diagram have been determined. From EMG values, activities of Al with respect to mole fraction of Al in ternary Al-Cu-Zn alloys for three quasi-binary sections (x(Cu)/x(Zn) = 7/3, 1, and 3/7) at 850-degrees-C were determined. The negative deviation from Raoultian behavior observed was more pronounced at high x(Cu)/x(Zn), ratios in agreement with binary Al-Zn and Al-Cu phase diagrams.