The metastable phase equilibria for the ternary systems K2B4O7-K2SO4-H2O and K2B4O7-KCl-H2O at 273 K were studied using an isothermal evaporation crystallization method. The solubilities of salts and densities of solution in these systems were determined. According to the experimental data and the corresponding equilibrium solid phases, the metastable phase diagrams of the ternary systems K2B4O7-K2SO4-H2O and K2B4O7-KCl-H2O at 273 K were drawn and the density diagrams were constructed, respectively. Experimental results show that the two phase diagrams are both constituted of a eutectic point, two univariate solubility curves and two solid phase crystallization regions. The equilibrium solid phases at the eutectic point are K2SO4 and K2B4O7 center dot 4H(2)O in the ternary system K2B4O7-K2SO4-H2O, and the equilibrium solid phases at the eutectic point are KCl and K(2)B(4)O(7 center dot)4H(2)O in the ternary system K2B4O7-KCl-H2O at 273 K. Therefore, the metastable phase diagrams for ternary systems K2B4O7-K2SO4-H2O and K2B4O7-KCl-H2O at 273 K both belong to a simple co-saturated type and neither double salt nor solid solution was formed.