The solid-liquid phase equilibria of the ternary system H2O-NH4H2PO4-(NH4)(2)HPO4 were studied under air pressure using a synthetic method based on conductivity measurements. Two isotherms were set at -7 and -9 degrees C. Two branches of solubility, stable and metastable, have been identified. The solid phases which lay out stable equilibria with the liquid are: alpha-NH4H2PO4, beta-NH4H2PO4, (NH4)(2)HPO4 center dot 2H(2)O and ice. Only (NH4)(2)HPO4, which is a solid phase, makes up a metastable equilibrium at this temperature. (C) 2013 Published by Elsevier B.V.