Solid-liquid phase equilibria of the quaternary system (Li + K + Mg + SO4 + H2O) at 288.15 K were investigated with the isothermal dissolution method. The diagrams of solubilities, densities and refractive indices were plotted, respectively. The solubilities diagram of the quaternary system includes three invariant points cosaturated with three salts and five crystallization zones. Two kinds of double salts K2(S)O(4)center dot MgSO4 center dot 6H(2)O and Li2SO4 center dot K2SO4 were found, and no solid solution was formed. Pitzer theory for aqueous electrolyte solution and its extended Harvie-Moller-Weare (HMW) model was used to calculate the solubilities of the quaternary system. A comparison between the calculated and the experimental results shows that the predicted solubilities obtained with the extended HMW model agree well with experimental data.