The effect of additives on the LCST phase behavior of aqueous solutions of either poly(N-isopropylacrylamide) (PNIPAM) or poly(vinyl methyl ether) (PVME) has been investigated using high-resolution ultrasonic spectroscopy (HR-US) and modulated temperature differential scanning calorimetry (MTDSC). Both techniques revealed that the addition of salt causes a decrease in demixing temperature (T-demix) due to the water-structuring capacity of salt ions. This salting-out effect becomes more pronounced at high polymer concentration, causing an asymmetric shape of the LCST demixing curve. Conversely, adding a surfactant results in an increase of T-demix because of the increased solubilization of the polymer chains. In addition, HR-US provides supplementary information on a molecular level, illustrating that both types of additives dissimilarly affect the polymer-water hydration structure; i.e., salt ions primarily dislocate the structured water molecules, whereas surfactants interact with the polymer itself.