Osmotic coefficients, water activities, and vapor pressures for the systems of LiCl + H2O, MgSO4 + H2O, and LiCl + MgSO4 + H2O over the ionic strength ranges from I = (0.5552 to 7.0004, 4.1028 to 22.6048, and 1.1982 to 20.5863) mol.kg(-1) respectively at T = (273.15, 298.15, 323.15, 348.15, and 373.15) K were determined by the isopiestic method with an improved simple apparatus. Aqueous CaCl2 solution was chosen as a reference standard. The measured osmotic coefficients for the binary solutions of LiCl + H2O and MgSO4 + H2O were in agreement with those from the literature. The experimental data of osmotic coefficients for the ternary mixtures were represented by using the Pitzer ion -interaction model; the expressions for the temperature dependencies of the model parameters were given. The standard deviations of all the measured osmotic coefficient data from those calculated by using the model over the temperature range were estimated. A set of model equations of apparent molar enthalpy and excess heat capacity for the mixtures as functions of molality and temperature were obtained by differentiating the model equations of excess Gibbs free energy. The mean activity coefficients, relative apparent molar enthalpies, and excess heat capacities were calculated by using the corresponding equations. The effects of composition and temperature on these thermodynamic properties were discussed.