In this paper, we present a methodology for the prediction of the crystallization enthalpy of mirabilite (Na2SO4. 10H(2)O) from supersaturated H2O-NaCl-Na2SO4 solutions. Required thermodynamic properties of H2O-NaCl-Na2SO4 such as dissolution enthalpies, phase equilibria and heat of hydration of Na2SO4 are represented or determined using Fitter's ion-interaction model. Measurements of crystallization enthalpies at various temperatures and concentrations were made in a SETARAM C80D calorimeter by mixing H2O-Na2SO4 and H2O-NaCl-Na2SO4 solutions supersaturated relatively to mirabilite with a seed crystal of Na2SO4. 10H(2)O once the thermal equilibrium was reached. Good agreement was obtained between experimental and predicted values of crystallization enthalpies. Furthermore, we have studied the influence of NaCl used as an additive regarding the heat storage capacity of supersaturated H2O-Na2SO4 solutions.