The solubility of the aqueous system containing lithium, sodium, potassium, chloride, and borate ions and the densities, viscosities, refractive indexes, conductivities, and pH at 298.15 K of the system have been studied. On the basis of the Janeche index values, the phase diagrams of dry salt, sodium, and water of the system were plotted. The dry-salt phase diagram of the system includes two four-salt cosaturation points, five solubility isotherm curves, and four crystallization zones all saturated with sodium chloride corresponding to potassium chloride, lithium chloride monohydrate, borax, and lithium borate trihydrate. No solid solution and double salts were found. On the basis of Pitzer's semiempirical equation for a mixed aqueous electrolytic solution, the chemical equilibrium model of Harvie was used to fit the single-salt Pitzer parameters of lithium borate, beta((0)), beta((1)), and C-phi and the triplet mixed parameters Psi(Li,Cl,B45(OH)4), Psi(Li,K,B4O5(OH)4), and Psi(Li,Na,B4O5(OH)4) in this system, which are not reported in the literature. The prediction of the solubilities of salts in the five-component system was then demonstrated. The calculated solubilities agree well with the experimental values.