Magnesium hypophosphite Mg(H2PO2)(2) is an important hypophosphite, and used as an drugs for the treatment of rheumatoid arthritis, plastic stabilizer, flameresistant material, fertilizer additive and the amendment of the soil, which can be synthesized via neutralization reaction and double decomposition reactions. In this work, the (solid + liquid) phase diagrams of (Mg(H2PO2)(2) + H2O) binary systems, and (Mg(H2PO2)(2) + NaH2PO2 + H2O), (Mg(H2PO2)(2) + MgCl2 + H2O) ternary systems at 298.15 K were experimentally determined via the classical isothermal solubility equilibrium method. It was found that (1) two solid salts of Mg(H2PO2)(2)center dot 6H(2)O and Mg(H2PO2)(2) with transition temperature 325.15 K occur in the (Mg(H2PO2)(2) + H2O) binary system, (2) three solid salts of Mg(H2PO2)(2)center dot 6H(2)O, NaH2PO2 center dot H2O and one incompatible double salt [NaMg(H2PO2)(3)] exist in (Mg(H2PO2)(2) + NaH2PO2 + H2O) system, (3) Mg(H2PO2)(2)center dot 6H(2)O, MgCl2 center dot 6H(2)O and one unknown double salt exist in (Mg(H2PO2)(2) + MgCl2 + H2O) system, the unknown double was identified to be an incompatible double salt [Mg-2(H2PO2)(2)Cl-2 center dot 6H(2)O]. (C) 2015 Elsevier B.V. All rights reserved.