We have studied the state of the water contained in poly(ethylene terephthalate) (PET), which consists of repeat units of OCC6H4COOCH2CH2O, in variously hydrated states. We first determined the hydration structure of the water therein not only from its OH stretching spectrum in a thinner sample but also from the hydration energy, the effect of the hydrogen bonding on the lengths of the donor and acceptor bonds, and the OH stretching frequencies of the water for the optimized 1:1 hydrate structures (quantum-chemically calculated). It has been found that the water bridges two ester C=O's in the manner of C=O center dot center dot center dot H-O-H center dot center dot center dot O=C therein and that about a 0.05 mole fraction of the C=O groups is bridged by the water in a PET sample hydrated in open air. We then carefully analyzed the state of the hydrating water in a thicker sample from its combination band around 5240 cm(-1), which significantly changes in frequency and bandwidth depending on the quantity of the contained water. It has been shown that the hydrating water molecules are so mobile as to begin to intermolecularly interact among themselves at a low hydration density of 10-15 water molecules per 1000 repeat units of OCC6H4COOCH2CH2O in the solid matrix.