Phase equilibria for the reciprocal quaternary system containing rubidium, magnesium, chloride, and borate in aqueous solution at 348 K was investigated by isothermal dissolution method. The compositions, densities, and refractive indices of the solution at equilibrium were measured experimentally. The space phase diagram, the planar projection diagram, the water content diagram, and the diagrams of the physicochemical properties (densities and refractive indices) vs composition were constructed using the measured data. Results indicate that the quatemary system is a complex type along with the double salt rubidium carnallite (RbCl center dot MgC12 center dot 6H(2)O) formed at 348 K. The planar projection diagram consists of three invariant points, seven univariant curves and five crystallization zones corresponding to four single salts rubidium pentaborate tetrahydrate (RbB5O6(OH)(4)center dot 2H(2)O), hungchaoite (MgB4O3(OH)(4)center dot 7H(2)O), rubidium chloride(RbCl), bischofite(MgC12 center dot 6H(2)O), and a double salt of rubidium carnallite (RbCl center dot MgCl2 center dot 6H(2)O). With a view to the crystallization zones, the crystallization zone of salt RbB5O6(OH)(4)center dot 2H(2)O occupies the largest part, meaning rubidium borate can be more easily separated from solution than the other coexisting salts in this system at 348 K. The water content and physicochemical properties of the equilibrium solution corresponding the univariant curve EH3 change obviously with the increase of J(Mg2+), whereas the water content and physicochemical properties change only slightly on the other univariant curves.