An experimental study on phase equilibria at 288 K in the quaternary system Na2CO3 + K2B4O7 + K2CO3 + Na2B4O7 + H2O was done by an isothermal solution saturation method. Equilibrium solubilities and properties such as the density, electrical conductivity, and pH of the solution were determined experimentally. According to the experimental data, the equilibrium phase diagram was constructed in which there were three invariant points, seven univariant curves, and five crystallization fields (K2CO3.3/2H(2)O, K2B4O7.4H(2)O, Na2B4O7.10H(2)O, Na2CO3.10H(2)O, NaKCO3.6H(2)O). The experimental results were discussed briefly.