The present study investigates the metastable phase equilibria of ternary systems KCl-MgCl2-H2O and MgB4O7-MgCl2H2O at 273 K by an isothermal evaporation method. The solubility data of the two ternary systems were determined. According to the solubility data, the corresponding metastable phase diagrams were plotted. The equilibrium solid phases of invariant points were identified by X-ray powder diffractometry. The results show that the ternary system KCl-MgCl2H2O has a double salt KCl center dot MgCl2 center dot 6H(2)O. Its metastable phase diagram contains three univariate curves, two invariant points and three crystallization areas, which correspond to MgCl2 center dot 6H(2)O, KCl and KCl center dot MgCl2 center dot 6H(2)O. Compared with the stable phase diagram at 273 K, the unsaturated phase region increases slightly in the metastable phase diagram. The ternary system MgB4O7-MgCl2-H2O is of simple co-saturated type without double salt or solid solution. The metastable phase diagram contains two univariate curves, one invariant point and two crystallization fields, and the corresponding solid phases are MgCl2 center dot 6H(2)O and MgB4O7 center dot 9H(2)O. Comparing the metastable diagram against the stable phase diagram at the same temperature, it can be seen that the solubility of MgB4O7 in the metastable state obviously increased, which means that magnesium borate was significantly supersaturated under the metastable state.