An experimental study on phase equilibria at 288 K in the quinary system Li+ + Na+ + K+ + SO42- + B4O72- + H2O was performed using the isothermal evaporation method. Equilibrium solubility and density of the solution were measured. The equilibrium phase diagram for the solution saturated with the salt Li2B2O4 center dot 16H(2)O was constructed. The phase diagram of this system consists of 7 invariant points, 14 univariant curves, and 8 crystallization fields corresponding to potassium tetraborate pentahydrate (K2B4O7 center dot 5H(2)O), borax (Na2B4O7 center dot 10H(2)O), potassium sulfate, lithium sulfate monohydrate (Li2SO4 center dot H2O), sodium sulfate, lithium-potassium sulfate double salt (Li2SO4 center dot K2SO4), lithium-sodium sulfate double salt (Li2SO4 center dot Na2SO4), and potassium-sodium sulfate double salt (3K(2)SO(4)center dot Na2SO4). There are three kinds of double salts formed in the system. The crystallization forms of lithium and potassium borate were K2B4O7 center dot 5H(2)O and Li2B2O4 center dot 16H(2)O, respectively, which are different from the usual forms of K2B4O7 center dot 4H(2)O and Li2B4O7 center dot 3H(2)O that occur at stable equilibrium.