Phase equilibria for two ternary systems K+, NH4+//Cl--H2O, and Mg2+, NH4+/Cl--H2O at T = 273 K were investigated by means of the isothermal saturation dissolution method. The density of each equilibrium liquid phase was determined by using the weighting bottle method. The compositions of the solid phases were confirmed using Scherinemakers' wet residue method. The phase diagrams and the figures of density versus composition were plotted in accordance with the experimental data. The ternary system Mg2+, NH4+/Cl--H2O is a type of complex with an incongruent type complex salt ammonium carnallite (NH4Cl center dot MgCl2 center dot 6H(2)O) formed at 273 K, which consists of two invariant points, three monovariant curves, and three solid phase crystallization fields. The ternary system of K+, NH4+//Cl--H2O consists of three invariant points, four monovariant curves, and five solid phase crystallization fields and is also a type of complex with two solid solutions [(K, NH4)Cl] and [(NH4, K)Cl] found at 273 K. The comparison of obtained phase diagrams at 273 K in this work and 298 K has been deliberated.