The emf of galvanic cells with zirconia solid electrolytes was measured to determine the activity of gallium in liquid Ga-Sb-Sn alloys in the 1050-1150 K temperature range along three lines of constant antimony-to-tin mole ratio. A mixture of Ga and Ga2O3 was used as a reference electrode. The activity curves of Ga show negative to positive deviations from the ideal behavior with increasing Sn concentrations in the lower concentration range of Ga; whereas in the concentration range X(Ga)greater than or equal to 0.6, the activity of Ga in the ternary alloys shows positive deviations from the ideal behavior. Isoactivity curves at 1073 K, in the ternary Ga-Sb-Sn alloys, were derived by combining the activity data for Ga-Sb and Ga-Sn alloys. The shape of the curves is simple in the Ga-poor region, but shows a wavy form with increasing Ga concentration. Isovalue curves in the excess free energies of mixing in the ternary alloys are derived by the Darken method. The results are in agreement with values derived by using the Chou method on the Sb-poor side, but the agreement is not good in the Sb-rich region.