Abstract: Aimed at thermodynamic study, isopiestic experiments were carried out at 298.15 K for the quinary aqueous solution (water + potassium chloride + cadmium chloride + mannitol + sorbitol) and its two quaternary sub-systems (water + potassium chloride + cadmium chloride + mannitol) and (water + potassium chloride + cadmium chloride + sorbitol) to equilibrate them with a solid complex containing cadmium chloride and potassium chloride, with sodium chloride (aq) taken as reference solution to determine the osmotic coefficients of the quinary aqueous solution at different aqua activities. The results showed that the isopiestic behavior of the quinary system related to its quaternary sub-systems are well represented by the ideal-like solution model within permissible error range.