In this work, the solid-liquid phase equilibria and physicochemical properties of ((NH4)(2)SO4-MnSO4-H2O) and ((NH4)(2)SO4-MgSO4-H2O) systems at T = 303.15 K have been determined by the isothermal solution saturation method. Also we used the Schreinemaker wet residue method to determine the equilibria solid phase. The experimental results show that there are two double-salts, (NH4)(2)Mn(SO4)(2)center dot 6H(2)O and (NH4)(2)Mg(SO4)(2)center dot 6H(2)O, in both systems. At the same time, there are two common points ((MnSO4 center dot H2O+(NH4)(2)Mn(SO4)(2)center dot 6H(2)O) and ((NH4)(2)SO4+(NH4)(2)Mn-(SO4)(2)center dot 6H(2)O) in the (NH4)(2)SO4-MnSO4-H2O system, (MgSO4 center dot 6H(2)O+(NH4)(2)Mg(SO4)(2)center dot 6H(2)O) and ((NH4)(2)SO4+(NH4)(2)Mg(SO4)(2)center dot 6H(2)O) in the (NH4)(2)Mg(SO4)(2)center dot 6H(2)O system) and five crystalline regions. The density and refractive index of the two systems are calculated and compared with the experimental values. According to means of the through and intermediate translation techniques together, we predicted the phase diagram of the (NH4)(2)SO4-MnSO4-MgSO4-H2O quaternion system. Thus, the study of solid-liquid phase equilibrium at 303.15 K is of great theoretical significance to the crystallization process of manganese sulfate.