Phase equilibria in the ternary systems K2SO4-K2B4O7-H2O and K2SO4-Na2B4O7-H2O at 348 K were studied by the isothermal solution saturation method. The solubilities and densities of the solution in these ternary systems were determined experimentally. Using the experimental data, the phase diagrams of the two ternary systems were obtained. The ternary systems K2SO4-K2B4O7-H2O and K2SO4-K2B4O7-H2O at 348 K are a type of simple common saturation and without complex salt and solid solution. In the phase diagrams and both the two ternary systems at 348 K, all have one invariant point, two univariant curves, and two regions of crystallization. The two crystallization regions of the ternary system K2SO4-K2B4O7-H2O at 348 K correspond to potassium borate tetrahydrate (K2B4O7 center dot 4H(2)O) and potassium sulfate (K2SO4). The two single-salt crystallization regions are sodium borate pentahydrate (Na2B4O7 center dot 5H(2)O) and sodium sulfate (Na2SO4) in the ternary system K2SO4-K2B4O7-H2O at 348 K.