To elaborate a new method of synthesis of barium hypophosphite based on an exchange reaction, the solubility in the Na₂Cl₂–Na₂(H₂PO₂)₂–Ba(H₂PO₂)₂–H₂O system has been investigated by isothermal method at 0⁰C.The solubility and the phase equilibrium of the quaternary water-salt system of Na₂Cl₂–Na₂(H₂PO₂)₂–Ba(H₂PO₂)₂–H₂O which is covered by the quaternary alternate water-salt system of Na⁺, Ba⁺²//Cl^-, H₂PO₂^-//H₂O were studied. The combination at the eutonic point of the studied quart system was found (as percent of mass) like NaCl – 1.06, NaH₂PO₂ – 43.98, Ba(H₂PO₂)₂ – 0.51 and H₂O – 54.45. It was determined that the liquid phase and the crystals of NaCl, NaH₂PO₂∙H₂O and Ba(H₂PO₂)₂∙H₂O were on an even keel at that eutonic point. It was established that the Ba(H₂PO₂)₂∙H₂O crystals had a room of 87.1 percent in the total crystallization area