Utilizing improvements in experimental equipment, analytical methods and the initial material, the solubility isotherms of the ternary system MgCl2-MgSO4-H2O were determined in detail at T = (323.15 and 348.15) K using an isothermal method of solid liquid equilibrium. The results indicate that the solid phases MgSO4 center dot nH(2)O((s)) (n = 6, 1) and MgCl2 center dot 6H(2)O((s)) are stable and MgSO4 center dot nH(2)O((s)) (n = 5, 4) are metastable at 323.15 K, which contradicts the results of a previous experimental study(1) in which the phase MgSO4 center dot 4H(2)O((s)) was reported as stable. The liquidus of the four solid phases MgSO4 center dot nH(2)O((s)) (n = 6, 4, 1) and MgCl2 center dot 6H(2)O((s)) were measured at 348.15 K in this work. The findings indicate that the phases MgSO4 center dot H2O(s) and MgCl2. 6H(2)O((s)) are stable and MgSO4-6H(2)O(s) and MgSO4 center dot 4H(2)O((s)) are metastable at 348.15 K. Remarkable differences between this work and the literature solubility data for the phase MgSO4 center dot H2O(s) at 348.15 K are observed. A Pitzer-Simonson-Clegg thermodynamic model was chosen to simulate the properties of the binary systems and to correlate the solubility isotherms of the ternary system at 298.15 K in our previous study and 323.15 K and 348.15 K in this work. Good agreement has been found between the calculated and experimental results. Applying the model parameters and solubility isotherms in the ternary system measured in this work, we obtained the solubility product parameters In K and chemical potentials for the solid phases MgSO4 center dot nH(2)O((s)) (n = 6, 5, 4, 1) over a wider temperature range than those for the binary system MgSO4-H2O.