The solid-liquid equilibrium (SLE) of electrolyte mixtures Li+, K+, Rb+//borate-H2O at T = 323 K was studied by the isothermal dissolution method, and the X-ray diffraction method was used to confirm the solid-phase compositions. The analysis of phase diagram shows that the graph consists of one three-salt cosaturation invariant point and three regions of crystallization corresponding to single salts Li2B4O7 center dot 3H(2)O, K2B4O7 center dot 4H(2)O, and RbB5O8 center dot 4H(2)O, respectively. From the comparison of phase diagrams of system Li+, K+, Rb+//B4O72--H2O between 323 and 348 K, we can conclude that (1) the crystalline form of salts lithium borate, potassium borate, and rubidium borate did not change at 323 and 348 K and (2) the salt Li2B4O7 center dot 3H(2)O crystalline area decreases and the other two salt crystalline areas, K2B4O7 center dot 4H(2)O, and RbB5O8 center dot 4H(2)O, increase at 323 K.