The solubilities of ternary system Na2SO4-CO(NH2)(2)-H2O and MgSO4-CO(NH2)(2)-H2O and quaternary system Na2SO4-MgSO4-CO(NH2)(2)-H2O at 60 degrees C were measured by isothermal method, and the corresponding phase diagrams were studied. There are five crystalline fields in the quaternary system Na2SO4-MgSO4-CO(NH2)(2)-H2O at 60 degrees C, which correspond to MgSO4 center dot 6H(2)O, Na2SO4 center dot MgSO4 center dot 4H(2)O, Na2SO4, CO(NH2)(2) and MgSO4 center dot CO(NH2)(2)center dot 4H(2)O respectively. On the basis of the analyses of the phase diagram of the quaternary system Na2SO4-MgSO4-CO(NH2)(2)-H2O at 60 degrees C and 25 degrees C, Na2SO4 and Mg-N compound fertilizers can be produced using CO(NH2)(2) as salting -out agent directly. On the basis of the extended Pitzer model of electrolyte solution theory, the solubilities of Na2SO4-MgSO4-CO(NH2)(2)-H2O system at 60 degrees C were calculated. The results showed that calculated values were well consistent with experimental results. (C) 2015 Published by Elsevier B.V.