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The thermal behavior of PNIPAM in its concentrated D₂O solution (20 wt %) was studied by FTIR and 2D-IR correlation spectroscopy. The spectral data of the C−H groups and the Amide I region provide details about the changes of the hydrophobic and hydrophilic parts in the polymer respectively during a heating−cooling cycle. The reversal of peak positions of the C−H bands upon cooling indicates the reversibility of temperature-induced dehydration of the hydrophobic groups. The change in hydrogen bonding of CO···D−N constructed between dehydrated CO and N−D groups, as derived from the Amide I region, does not revert precisely in the cooling process due to the newly formed hydrogen bonds in the collapsed state, and a hysteresis phenomenon is observed. In our concentrated solution (20 wt %), the strength of those intra- and interchain hydrogen bonds even prevent the polymers from dissociating …