The study of the ternary system H2O-NH4NO3-Mg(NO3)(2) has been simplified by research of a quasi binary section NH4NO3-Mg(NO3)(2). 6H(2)O which limits the quasi ternary system H2O-NH4NO3-Mg(NO3)(2). 6H(2)O. The isotherm 60 degrees C of the quasi ternary system has been established entirely by conductimetric measurements. The solid phases in equilibrium with the saturated solution are the hexahydrate alpha of the magnesium nitrate and a solid solution of the variety III of the ammonium nitrate. The exploitation of experimental results obtained by direct thermal analysis has allowed to draw the quasi binary system NH4NO3-Mg(NO3)(2). 6H(2)O and to establish the full polythermic phase diagram of the system H2O-NH4NO3-Mg(NO3)(2). 6H(2)O.