The hydration properties of poly(N-isopropylacrylamide) in aqueous solution were investigated by Fourier transform infrared spectroscopy as a function of high hydrostatic pressure and compared to the thermally induced changes. We show that although both pressure and temperature induce a phase separation the underlying mechanisms are fundamentally different. It is well documented that increasing the temperature above the lower critical solution temperature causes a dehydration of the hydrophilic and hydrophobic moieties. By contrast, high pressure enhances the hydration of the hydrophilic amide group. Moreover, pressure strengthens the weak C-H center dot center dot center dot O hydrogen bonds between the hydrophobic alkyl groups and water, although a reorganization of the water network around the hydrophobic groups occurs during the phase separation. From this it is concluded that PNiPA remains in a coillike state at high pressure. In addition, we suggest that PNiPA is a good model for the study of the hydration properties of proteins.