The stable solid-liquid phase equilibrium in the ternary system MgSO4. + MgCl2 + H2O at 263.15 K was investigated with the isothermal dissolution method, and the phase diagram was plotted. Two isothermal invariant points cosaturated with MgSO4 center dot 7H(2)O + ice and MgCl2 center dot 6H(2)O + MgSO4 center dot 7H(2)O, three stable crystallization fields corresponding to V (Bis), MgSO4 center dot 7H(2)O (Eps), and ice, and three isothermal univariant curves were determined in the ternary system. Compared to the phase equilibrium at 273.15 K, the ice crystallization region and ice-salt invariant point are found at 263.15 K. Neither solid solution nor double salts are found in the system.