The stable phase equilibria of quaternary systems LiBr-NaBr-MgBr2-H2O and LiBr-KBr-MgBr2-H2O at 298.15 K were studied by both experimental measurement(isothermal solution saturation method)and theoretical prediction(Pitzer model).The solubilities of the saturated solution have been determined experimentally and two sta-ble phase diagrams and relevant water diagrams of the two quaternary systems were obtained.Results show that qua-ternary system LiBr-NaBr-MgBr2-H2O is hydrate Ⅱ type as NaBr and NaBr·2H2O coexistence.Its phase diagram consists of only one invariant point,four univariant curves,and five crystallization fields.The quaternary system LiBr-KBr-MgBr2-H2O is a complex type as the double salt KBr·MgBr2·6H2O formed.In addition to this double salt,the three single salts LiBr·2H2O,KBr and MgBr2·6H2O also crystallize.In this paper,the solubilities of phase equilibria in above quaternary systems were also calculated by the Pitzer’s electrolyte solution model.All the needed parameters can be obtained from the literature or be fitted by experimental data.On the Basis of the experimental and calculated results,the phase diagram of the quaternary system was plotted for comparison.It shows that the calculation results are consistent with the experimental ones.