The thermodynamic properties of the lithium‐gallium binary system have been investigated by the use of electrochemical methods, with special attention being given to the phase . Measurements have also been made of the mass transport parameters in , including the determination of the composition dependence of the chemical diffusion coefficient, the self‐diffusion coefficient, and the enhancement factor. The data on emf vs. composition, obtained by use of the coulometric titration technique at 415°C, show that all three intermediate phases, whose nominal compositions are , , and , have appreciable ranges of stoichiometry. The existence of these three phases was further substantiated by x‐ray diffraction analysis. At 415°C, the constant emf values in the two‐phase regions are 565, 122, 91, and 20 mV with respect to pure liquid lithium, in the order of increasing lithium concentration. Measurements are reported on the temperature dependence of the most gallium‐rich plateau, leading to values of the partial molar entropy and enthalpy of lithium in that two‐phase region. The lithium activity has been found to follow Henry's law in liquid lithium‐gallium alloys and electrochemical measurements of the temperature dependence of the solubility of lithium in liquid gallium confirmed earlier results. The activity of lithium in the vario‐stoichiometric phase lies between and 0.126 at 415°C over a composition range from 44.8 to 56.0 a/o Li. The Gibbs free energy of mixing was determined as a function of composition across the phase and found to have a minimum value of −25.8 kJ/mol at 47.6 a/o Li. The standard Gibbs free energy of formation of all three intermediate phases was evaluated at their nominal stoichiometric compositions; the values are −51, −115, and −62 kJ/mol, respectively. The chemical diffusion coefficient within the phase varies from . On the lithium‐rich side of , the chemical diffusion coefficient increases slightly with increasing lithium concentration. On the lithium‐deficient side of the ideal stoichiometry, the chemical diffusion coefficient increases rapidly as lithium is deleted. After reaching a maximum value at 47.6 a/o Li, it gradually decreases again with further decrease in lithium content. The enhancement factor, which relates the chemical and self‐diffusion coefficients, is also composition dependent. It reaches a maximum value of 56 at the same composition as the peak in the chemical diffusion coefficient and the minimum in the Gibbs free energy of mixing.