Activities of gallium in the liquid Au-Ga alloys were determined using galvanic cells with the solid oxide electrolyte. The cells of the type: $${\text{Au}}_{x} {\text{Ga}}_{(1 - x)} //{\text{ ZrO}}_{2} + {\text{Y}}_{2} {\text{O}}_{3} //{\text{ Fe}},\,{\text{FeO}}$$ were used in the temperature range from 1050 to 1273 K, and for four alloy compositions x_Ga = 0.2, 0.4, 0.6 and 0.8. From the measured e.m.f.’s the activity of gallium was derived, which taken together with the recently measured heat of mixing provided precise description of the thermodynamic properties of the liquid phase. This study was supplemented with DTA experiments which were performed for three alloy compositions, namely 0.15, 0.25 and 0.40 gallium mole fraction. The results of these experiments verified with temperature of respective invariant reactions. Finally, present results, taken together with the information existing in the literature, were used for the calculation of the optimized Au-Ga phase diagram. ThermoCalc software was applied to complete these calculations.