The solubility values and physicochemical properties (pH values, refractive indices, and densities) of electrolyte mixtures in quaternary system containing lithium, potassium, rubidium, and borate were investigated at 348 K by isothermal dissolution method. The space phase diagram, the planar projection diagram, the water content diagram, and the diagrams of physicochemical properties depending on the composition were obtained using the measured data. The phase diagram of this quaternary system contains one invariant point, three univariant curves, and three single salts corresponding to lithium tetraborate trihydrate (Li2B4O7 center dot 3H(2)O), potassium tetraborate tetrahydrate (K2B4O5(OH)(4)center dot 2H(2)O), and rubidium pentaborate tetrahydrate (RbB5O6(OH)(4)center dot 2H(2)O). This quaternary system is of a simple cosaturation type, no double salt or solid solution formed. The scope of areas of crystallization of salts is such that RbB5O6(OH)(4)center dot 2H(2)O > Li2B4O7 center dot 3H(2)O > K2B4O5(OH)(4)center dot 2H(2)O, which demonstrates the salt RbB5O6(OH)(4)center dot 2H(2)O can be more easily separated from the solution than the other coexisting salts at 348 K. The physicochemical properties of the solutions at equilibrium on the univariant curve CE increase obviously with increasing Janecke index, J(K2B4O7); whereas on the univariant curve AE or BE, the physicochemical properties change slightly along with the changes of J(K2B4O7).