An experimental study on phase equilibria at 288 K in the quinary system Li+ + Na+ + K+ + CO32- + B4O72- + H2O was done by the isothermal solution saturation method. Equilibrium solubilities and properties such as the density, electrical conductivity, and pH of the equilibrium solution were determined experimentally. According to the experimental data, the equilibrium phase diagram saturated with Li2CO3 was constructed. The phase diagram of the quinary system has four invariant points, nine univariant curves, and six crystallization fields (K(2)CO(3)center dot 3/2H(2)O, K(2)B(4)O(7)center dot 4H(2)O, Na(2)B(4)O(7)center dot 10H(2)O, Na(2)CO(3)center dot 10H(2)O , NaKCO(3)center dot 6H(2)O, and Li(2)B(4)O(7)center dot 3H(2)O). Potassium carbonate (K(2)CO(3)center dot 3/2H(2)O) has the smallest crystallization field. The borates (K(2)B(4)O(7)center dot 4H(2)O, Li(2)B(4)O(7)center dot 3H(2)O, and Na(2)B(4)O(7)center dot 10H(2)O) have larger crystallization fields than the others. The double salt NaKCO(3)center dot 6H(2)O was found in the quinary system.