The phase transitions of poly(vinyl methyl ether) (PVME) in H2O and D2O have been investigated by Fourier transform infrared (FTIR) spectroscopy. IR spectra of PVME solution are measured as a function of temperature, and changes in the spectra induced by the phase transition are observed. The IR bands attributable to the vibration modes of backbone and methoxy side group exhibit critical changes at a lower critical solution temperature (LCST). Heating of the solution above the LCST leads shifts of C-H stretching bands toward lower wavenumber during a high-wavenumber shift of C-O stretching band (nu (C-O)). Among the C-H stretching bands, symmetric and antisymmetric C-H stretching bands due to the methyl group (nu (s)(CH3) and nu (as)(CH3)) shift to the largest extent (20 cm(-1)). The nu (s)(CH3) band is composed of two components centered at 2839 and 2820 cm(-1), which are assignable to hydrated and dehydrated methyl species, respectively. The molar fraction of the hydrated methyl species is close to 1 below 36 degreesC and is nearly 0 above 40 degreesC, insisting that most of the methyl groups are dehydrated above the LCST, whereas the peak position of the nu (C-O) band indicates only partial dehydration of the ether group above the LCST. Though PVME concentration dependence of the peak positions of the nu (s)(CH3) and the nu (C-O) bands measured below the LCST shows that dehydration of the methyl group and the ether oxygen initiates at different water/monomer ratios (methyl, 5.5; ether, 2.8), the dehydration of these groups simultaneously proceeds in the course of the phase transition.