The terahertz attenuated total reflectance spectra of Poly(N-isopropylacrylamide) (P-NIPAAm) solutions were measured in the range of 26.5-47.3 degrees C. The phase-change in P-NIPAAm was observed via the temperature dependence of the spectra. The collapse of hydrogen bonds in solution was confirmed by the decrease in the signal intensities around 62 cm(-1), which corresponds to fast dielectric relaxation by rotation and collision of molecules, and around 166 cm(-1), which corresponds to the stretching mode of the intermolecular vibration of water molecules. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4711804]