In order to improve the technology for producing Mg-N compound fertilizer and NH4Cl based upon the analysis of phase diagram of Mg2+, NH4+//Cl-, SO42--H2O system at 25 degrees C, the phase equilibrium of this quaternary system at 0 degrees C was studied. The solubilities of the quaternary system Mg2+, NH4+//Cl-, SO42--H2O at 0 degrees C were measured by isothermal method, and the corresponding phase diagram was plotted. In this phase diagram, there are six solid phase crystalline zones, which correspond to MgCl2.6H(2)O, NH4Cl, ,MgCl2.NH4Cl.6H(2)O (NH4)(2)SO4, MgSO4-7H(2)O and MgSO4.(NH4)(2)SO4.6H(2)O respectively. The crystalline zones of MgSO4.(NH4)(2)SO4.6H(2)O and NH4Cl are larger than the others, which indicates that MgSO4. (NH4)(2)SO4. 6H(2)O and NH4Cl can crystallize out easily. According to the analysis and calculation of the phase diagrams of Mg2+, NH4+//Cl, SO42- -H2O system at 0 degrees C and 25 degrees C, the appropriate conditions for this technology were obtained. In the process, the Mg-N compound fertilizer can be prepared at 0 degrees C and the NH4Cl can crystallize out at 25 degrees C. The solubilities of the studied system were calculated on the basis of the extended Pitzer model, and the results showed that the calculated values agrees well with the experimental results. (C) 2015 Elsevier B.V. All rights reserved.