The solubilities and the physicochemical properties including the density, viscosity, refractive index, conductivity, and pH value of the aqueous quaternary system LiCl + KCl + Li2SO4 + K2SO4 + H2O at 308.15 K were determined with the isothermal evaporation method. On the basis of the Janecke indices, the dry-salt phase diagram, the water-phase diagram, and the diagrams of physicochemical properties were plotted. The dry-salt phase diagram of the system includes three three-salt cosaturation invariant points, seven univariant solubility curves, and five crystallization zones corresponding to potassium sulfate, potassium chloride, the double salt lithium potassium sulfate, lithium sulfate monohydrate, and lithium chloride monohydrate. Single-salt parameters beta((0)), beta((1)), and C-(phi) for lithium sulfate, potassium sulfate, and potassium chloride, mixed electrolyte parameters theta(Cl,SO4) and Psi(K,Cl,SO4,) and the Debye-Hukel limiting slope A(Phi) at 308.15 K were calculated according to the temperature-dependent equations presented in the literature. On the basis of Pitzer's semiempirical equations for a mixed aqueous electrolytic solution, the extended chemical equilibrium model of Harvie was used to fit the single-salt parameters beta((0)), beta((1)), and C-(phi) for lithium chloride, the mixing two-ion pair parameter of theta(Li,K), and the mixing triplet-ion parameters of Psi(Li,K,Cl) and Psi(Li,K,SO4) in this system at 308.15 K, which are not reported in the literature. The prediction solubilities for the quaternary system agree well with the experimental values.