New predictive equations have been established for thermal expansion, the apparent molal thermal expansion, speed of sound, and the apparent molal compressibility. These equations can provide the above thermodynamic properties for multicomponent solutions conforming to the linear isopiestic relation using only information on the corresponding binary sub-systems of equal water activity. No additional empirical parameters are involved. The predictive capabilities of the equations have been tested by comparisons with experimental data in the literature at 298.15 K and the values calculated by using Young's rule. The ternary systems used are NaCl-CaCl2-H2O, KCl-CaCl2-H2O, KCl-MOCl2-H2O, Na2SO4-MgCl2-H2O, and MgSO4-NaCl-H2O. The new equations in general provide better predictions and the agreement between the predicted and measured data is in general quite reasonable over the entire experimental composition ranges.