Hydration changes of poly(2-(2-methoxyethoxy)ethyl methacrylate) (PMoEoEMa) during thermosensitive phase separation in water have been investigated by infrared spectroscopy. The C=O stretching band can be separated into three components assigned to non-hydrated carbonyl groups and singly and doubly hydrogen-bonded carbonyl groups (1728, 1709, and 1685 cm(-1), respectively). Relatively large parts of the carbonyl groups (50% in 30 wt % solution) do not form hydrogen bonds even below the transition temperature (T-p) probably because they possess crowded positions near the backbone. The fraction of hydrogen-bonding carbonyl groups decreased during phase separation by similar to 0.2. Among five v(C-H) bands, the highest- and the lowest-frequency bands (v(C-H)A and v(C-H)E) exhibited relatively large red shifts of 8 and 11 cm(-1), respectively. DFT calculations indicate that the formation of a H-bond between the ether oxygen and water leads to blue shifts of v(C-H) of adjacent alkyl groups and has a larger effect. than a direct H-bond to the alkyl groups, namely, C-H center dot center dot center dot O H-bonds. The fraction of hydrogen-bonding methoxy oxygens estimated from the position of the v(C-H)(A) is 1 at < T-p and zero at > T-p. This result indicates that the methoxy oxygens and the carbonyl are more favorably hydrated than the other at < Tp and > T-p, respectively.