The solidliquid phase equilibrium in the reciprocal aqueous quaternary system Li+ , Mg2+ parallel to Cl , and borateH(2)O at 323 K was completed with the isothermal dissolution method. The corresponding physiochemical properties (rho, eta(D)) of solutions at equilibrium in the Li+, Mg2+parallel to Cl , and borateH(2)O electrolyte mixtures were determined. The isothermal dissolution phase diagram, the water figure, and the figures for the physicochemical properties (rho, eta(D)) depending on the composition were obtained according to values of the Janecke index. The stable phase diagram contains three invariant points, seven univariant curves, and five crystallization zones. The crystallographic forms of magnesium borate and lithium borate are tetraborate MgB4O5(OH)(4).7H(2)O and Li2B4O7.3H(2)O, respectively. The size of MgB4O5(OH)(4).7H(2)O crystallization field is the largest among the coexisting salts.