The separation of magnesium and aluminum bearing compounds from the (NH4)(2)SO4 aqueous solution is crucial for the ammonium sulfate roasting technique to extract Mg and Al from the magnesium aluminum-bearing slag. Equilibrium solubility data for the quaternary system of MgSO4 -Al-2(SO4)(3)-(NH4)(2)SO4-H2O at 40 and 80 degrees C were therefore measured and compared with those of their ternary subsystems of MgSO4-(NH4)(2)SO4-H2O and Al-2(SO4)(3)-(NH4)(2)SO4 H2O, respectively. The mutual salting-out effect between MgSO4 and Al-2(SO4)(3) in the quaternary system was further discussed. On the basis of the measured data, a strategy for effective separation of magnesium and aluminum compounds from the MgSO4-Al-2(SO4)(3)-(NH4)(2)SO4-H2O system was developed.