The solubility values and physicochemical properties such as densities, refractive indices, and pH values of the equilibrium solution in the quaternary system Li+, K+, Mg2+//borate-H2O at 348 K were measured by isothermal dissolution method. The phase diagram, water content diagram, and the diagrams of the physicochemical properties versus composition were constructed using the measured data. Results show that this quaternary system at 348 K is of a simple type, no double salt or solid solution formed. The stable phase diagram of this quaternary system consists of one invariant point, three univariant curves, and three crystalline phase areas. The invariant point saturated with three salts corresponding to lithium tetraborate trihydrate (Li(2)B(4)O7 center dot 3H(2)O), potassium tetraborate tetrahydrate (K(2)B4O(7)center dot 4H(2)O), and hungchaoite (MgB(4)O7 center dot 9H(2)O). The crystallization field of MgB(4)O7 center dot 9H(2)O is the maximum, meaning that the salt magnesium borate has the smallest solubility among the coexisting salts. Comparisons between the stable phase diagrams at 288 K and 348 K show that the crystallization zones of MgB(4)O7 center dot 9H(2)O decreased at 348 K, while the crystallization zones of K(2)B(4)O7 center dot 4H(2)O and Li(2)B(4)O7 center dot 3H(2)O both enlarged at 348 K. On the univariant curve CE, the water contents decrease with the increase of J(K2B4O7); the densities and refractive indices both increase with the increase of J(K(2)B4O(7)) and reach the maximum at invariant point E.