The solubility of the quaternary system of lithium, sodium, chloride, and carbonate and the densities, viscosities, refractive indexes, conductivities, and pH at 298.15 K have been studied. On the basis of the Janeche index values, the isothermal phase diagram of the system was plotted. The phase diagram of the system includes three invariant points, seven univariant curves, and five crystallization zones corresponding to sodium chloride, lithium chloride monohydrate, lithium carbonate, sodium carbonate heptahydrate, and natron (Na(2)CO(3)(.)10H(2)O). No solid solution and double salts were found. On the basis of Pitzer's semiempirical theory for mixed electrolytes, the chemical equilibrium model of Harvie was used for fitting the single-salt pitzer parameters of lithium carbonate, beta((0)), beta((1)), and C-phi, and two Pitzer's mixing triplet parameters, psi(Li,Na,CO3) and psi(Li,Cl,CO3), in this system, which are not reported in the literature. The prediction of solubilities for the quaternary system was then demonstrated. The calculated solubilities agree well with the experimental values.