The experimental study on phase equilibria at 258 K in the quinary system Na+, Mg2+//Cl-, NO3-, SO42--H2O and its subsystem Na+, Mg2+//Cl-, NO3--H2O were investigated using the isothermal solution equilibrium method. The phase diagrams were obtained on the basis of the measured data. The investigated systems belong to simple cosaturated types without double salts formed. The quinary system saturated with NaCl center dot 2H(2)O contains three isothermal-invariant points, seven univariant curves, and five crystallization fields corresponding to Na2SO4 center dot 10H(2)O, NaNO3, MgCl2 center dot 8H(2)O, MgSO4 center dot 7H(2)O, and Mg(NO3)(2)center dot 6H(2)O. With a view to the crystallization zones, the crystallization area of salt Na2SO4 center dot 10H(2)O occupies the largest part, meaning it can be more easily separated from the solution as compared to the other coexisting salts in this system. Comparison with the similar diagram type at 298 K shows that all fields, in general, have decreased except Na2SO4 center dot 10H(2)O; the main difference is that the crystallization fields of double salts disappear. These fundamental data could be valuable for promoting the research and application of crystallization processes at low temperature and further theoretical exploration.