The solubilities and densities of the solution in the quaternary system Na2B4O7-Na2SO4-K2B4O7-K2SO4-H2O at 288 K were investigated using the method of isothermal dissolution equilibrium. The solid liquid stable equilibria in the quaternary system Na2B4O7-Na2SO4-K2B4O7-K2SO4-H2O at 288 K were studied experimentally. On the basis of experimental data, the stable phase diagram of the quaternary system was plotted. In the solubility diagram of the quaternary system Na2B4O7-Na2SO4-K2B4O7-K2SO4-H2O at 288 K, there are three invariant points, seven solubility isotherm curves, and five crystallization regions, which correspond to borax (Na2B4O7 center dot 10H(2)O), potassium tetraborate (K2B4O7 center dot 4H(2)O), mirabilite (Na2SO4 center dot 10H(2)O), arcanite (K2SO4), and glaserite (Na2SO4 center dot 3K(2)SO(4)). It can be found from the experimental results that sulfates have bigger solubilities than borates in the quaternary system Na2B4O7-Na2SO4-K2B4O7-K2SO4-H2O at 288 K.