To understand the thermodynamic behaviors of the borate-containing system, the solid-liquid equilibrium data (solubilities, density, and refractive index) of quinary aqueous solution composed of lithium, potassium, rubidium, magnesium, and borate at 323.15 K were determined by using the isothermal dissolution method. The crystalloid forms of the solid phase were confirmed by the X-ray diffraction method. Results show that four single salts Li2B4O7 center dot 3H(2)O, K2B4O7 center dot 4H(2)O, RbB5O8 center dot 4H(2)O, and MgB4O7 center dot 9H(2)O were formed at 323.15 K in the quinary invariant point, with no double salt or solid solution found. Under the condition of Li2B4O7 saturation, the crystallization region of MgB4O7 center dot 9H(2)O occupied the largest crystallization region, and the salt K2B4O7 center dot 4H(2)O had the smallest region. Among the four salts, the crystalline regions of the salts MgB4O7 center dot 9H(2)O and RbB5O8 center dot 4H(2)O changed obviously at 323.15 and 348.15 K, that of MgB4O7 center dot 9H(2)O enlarged at 323.15 K, and those of RbB4O8 center dot 4H(2)O and K2B4O7 center dot 4H(2)O decreased.