The hydration structures of two different types of biomaterials, i.e., poly(2-methoxyethyl acrylate) (PMEA) and a random copolymer of 2-methacryloyloxyethyl phosphorylcholine and n-butyl methacrylate (P(MPC-r-BMA)), were investigated by means of attenuated total reflection infrared (ATR-IR) spectroscopy. The effects of the addition of sodium chloride to liquid water in contact with the surfaces of the polymer films were examined. The neutral polymer of PMEA was easily dehydrated by NaCl addition, whereas the zwitterionic polymer of P(MPC-r-BMA) was hardly dehydrated. More specifically, nonfreezing water having a strong interaction with the PMEA chain and freezing bound water having an intermediate interaction were hardly dehydrated by contacting with normal saline solution, whereas freezing water having a weak interaction with the PMEA chain was readily dehydrated. In contrast, freezing water in P(MPC-r-BMA) is exchanged for the saline solution contacting with the material surface without dehydration.