Solubility data for the quaternary system (Li2SO4 + Na2SO4 + MgSO4 + H2O) is very important for the separation of lithium sulfate from the salt lake brines in Qaidam Basin. The experimental and calculated solubilities of the quaternary system (Li2SO4 + Na2SO4 + MgSO4 + H2O) at T = 288.15 K and P = 0.1 MPa were determined. It is shown the solid phases Li2SO4 center dot H2O, Na2SO4 center dot 10H(2)O, and MgSO4 center dot 7H(2)O and a new complex salt Li2SO4 center dot 3Na(2)SO(4)center dot 12H(2)O exist in this system. The solubilities of this quaternary system were predicted using the Harvie-Moller-Weare (HMW) model of electrolyte ion-interaction theory. The modeling solubility isotherms agree well with experimental data, which indicates the solubilities and model parameters obtained in this work are reliable.