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
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