Poly(N-isopropylacrylamide) (NIPAAm) gels immersed in water (isobar gels) are known to undergo a volume phase transition at T-c congruent to 34 degrees C. This transition, due to the release of hydrophobic hydration, leads to dramatic changes in physical properties, such as mechanical, optical, and thermal properties. We dealt with NIPAAm isochore gels, where the gels were isolated from water to keep the gel concentration, and investigated the mechanical and thermal properties of the gels as functions of temperature, cross-link density, and polymer concentration. The dynamic storage modulus, E’, increased suddenly at T-c by a factor of 10 or more and became frequency dependent with increasing temperature. The enthalpy change related to the dissociation of the hydrophobic hydration was obtained as a function of the gel concentration. It was found that about 13 water molecules were released to dehydrate one NIP AAm molecule. Attenuated total reflection (ATR) infrared absorption spectroscopy was also conducted to elucidate the local environment of the NIPAAm gel network.