The activity coefficients of lithium chloride in aqueous solution at temperatures of 50, 75, 100, 125, and 150 °C and concentrations varying from 0.5 to 3 mhave been determined. The method used was the application of the Gibbs–Duhem theorem, i.e., the activity of the solvent water was the quantity actually determined. The experimental determination was that of the vapour pressure of the solution using a differential manometer. At all temperatures the activity coefficient passes through a minimum and then increases progressively.