Phase transitions of poly(N-vinylcaprolactam) (PVCL) and poly(N-vinylpyrrolidone) (PVPy) in H2O and D2O were investigated by turbidimetry, differential scanning calorimetry (DSC), and Fourier transform infrared (FTIR) spectroscopy. The phase diagram of PVCL solution is binodal with a lower critical solution temperature (LCST) of 32.5 degreesC and a critical concentration of about 5 wt %. PVPy also undergoes a phase separation around 30 degreesC in aqueous 1.5 M KF solution. The IR spectra of PVCL and PVPy in water critically change at the phase transition temperature (T-p). Above T-p the C-H stretching and the C-H bending bands shift to lower wavenumbers. The amide I band of PVCL is divided into four components, and the intensities of the 1565, 1588, and 1625 cm(-1) components increase and that of the 1610 cm(-1) component decreases while the phase transition proceeds. Addition of KCI and KF lower T-p of PVCL, whereas KI and KBr raise T-p. Addition of methanol gradually raises T-p up to ca. 40 vol % and steeply raises T-p at > 40 vol %. Although these salts do not alter the profiles of IR spectra of PVCL, methanol induces a change in the amide I band.