An experimental study of solid-liquid equilibria of H2O-Ba(NO3)(2) and H2O-Y(NO3)(3) systems has been conducted using conductimetry and controlled thermal flux analysis. The solid phases were studied using X-ray diffraction, thermogravimetry and chemical analysis such as calcination and DCP. Calorimetric analysis using DSC on the water-barium nitrate eutectic has given the transformation enthalpy (303 J g(-1)) and the specific heat capacities of the solid and the liquid mix between -20 and 15 degrees C. The invariant transformation temperatures of the H2O-Y(NO3)(3) system have been determined or qualified and the measured metastable melting point of the tetra-, penta- and hexahydrate are respectively 120.4, 77.8 and 51.8 degrees C. A thermodynamic study of the phase equilibria of the two systems has been achieved taking into account ionic dissociation and solvation phenomena. It shows that the barium nitrate in aqueous solution behaves as a binary electrolytic compound, that the yttrium nitrate in aqueous solution is dissociated and that the yttrium ion tends to solvate itself mainly into tetra-, octa- and dodecahydrate. The model used provides an excellent representation of the equilibrium curves. The metastable fusion enthalpies of the various hydrates have been computed and the validity check of these values has been successfully conducted on yttrium nitrate pentahydrate.