An experimental study on phase equilibria at 288 K in the quaternary system Na2SO4 + K2SO4 + Na2B4O7 + K2B4O7 + H2O was done by the isothermal evaporation method. Equilibrium solubilities and properties such as density, electrical conductivity, and pH of the solution were measured, and the equilibrium phase diagram was constructed in which there were two invariant points, six univariant curves, and four crystallization fields corresponding to borax (Na2B4O7• 10H(2)O), sodium sulfate, potassium sulfate, and potassium borate quahydrate (K2B4O7• 4H(2)O). No solid solutions or double salts were found.