With a fully automated high temperature calorimeter the enthalpy of formation of the [Ir+Ga] liquid system was determined between 1106<T e <1471 K, in the molar fraction range 0<x<0.54 (with x=x tr ). The molar enthalpy of formation of the [Ir+Ga] liquid alloys [Δ max H m °] corresponding to the reaction, at T e and p°: Ga(liq) + b lr(liq) → Ga, lr(1-a)(liq) can be described by the following Redlich-Kister equation (in kJ mol −1 ) Δ max H m °=x(1−x)ξ(y) with ξ(y)=−257.68−63.09y+76.47y 2 +14.39y 3 and y=x tr − x Ga . This function is negative with an estimated minimum Δ max H m °=−65±4 kJ mol −1 at x=0.55, and independent of temperature, within the experimental error. The limiting partial molar enthalpy of mixing of iridium, deduced from experiments performed at 1155 and 1482 K, is: Δ max h m °(Ir supercooled liquid in ∞ liq Ga) = − 132±5 kJ mol −1 . On the other hand, by extrapolation of the ξ-function to x = 1, the limiting enthalpy of Ga in supercooled liquid Ir was predicted with a larger uncertainty: Δ max h m °(Ga liq. in ∞ supercooled liquid Ir) = − 230±30 kJ mol −1 . For three compositions (x=0.14 1 , 0.18 2 and 0.25), the molar heat capacities of the solid alloys have been measured between 423 and 763 K and the enthalpy of formation of IrGa 3 was determined by dissolution calorimetry : Δ max H m °(Ir 0.25 Ga 0.75 ; T amb , sol) = −80 kJ mol −1 . Moreover, from these calorimetric experiments, some points of the equilibrium phase diagram were obtained; thus the first shape of the liquidus of the Ir + Ga system (in the Ga-rich region, has been proposed. The integral and limiting partial enthalpies of mixing have been compared with the data previously obtained for the (transition metal + Ga) systems.