[en] The solid-liquid equilibrium diagram of the system B₂O₃-Li₂O-H₂O is, up to now, known only in the temperature range 25 to 100 deg C. We determined the solubility isotherm in the system B₂O₃-Li₂O-H₂O at 300 deg C for B₂O₃/Li₂O mole ratio from 1 to 600 in solution. The values of saturation concentrations make it possible to predict the behaviour of a possible leak between primary and secondary circuit of nuclear reactors: the results show that at the beginning of the cycle, when the ratio B₂O₃/Li₂O is high, borate precipitation induced by boiling through the leak may lead to occlusion, and that this borate may dissolve when the ratio B₂O₃/Li₂O decreases during the course of the cycle. The solid phases identified by X-Ray diffraction were B₂O₃, Li₂B₁₀O₁₆H₂O, Li₄B₁₀O₁₇, Li₂B₄O₇, Li₃B₅O₈(OH)₂, LiBO₂. The existence of Li₃B₅O₈(OH)₂ was not reported in low temperatures equilibrium systems