Thermosensitive phase separation of aqueous solutions of poly(vinyl methyl ether) (PVME) has been investigated by infrared and Raman spectroscopy and differential scanning calorimetry (DSC) in the presence of metal halides as added salts. DSC measurements showed that the phase separation of PVME proceeds in two successive steps, that is, the first broad endothermic peak starting at T-p1 followed by the second sharp one at T-p2. KF, KBr, and KCl reduced T-p1 and T-p2 as well as the heat of phase transition, whereas KI raised these values. IR spectra showed that the C-H stretching (v(C-H)) band exhibited a red shift and the C-O stretching band (v(C-O)) exhibited a blue shift during the phase separation. Although addition of the salts altered T-p1 and T-p2, the profiles of the v(C-H) and v(C-O) bands measured in the presence of them were similar to those measured without salts. Confocal Raman microscopy reveled that droplets that are rich in polymer are formed above T-p1. The droplets showed a sharp decrease in PVME concentration at T-p2 and a gradual decrease between T-p2 and T-p1 at a cooling process. The temperature region where the CH2 stretching Raman band measured in the polymer-rich phase exhibited a positional change was lower than the region where the CH3 Stretching band exhibited a positional change, suggesting that the main chain of PVME is dehydrated at a lower temperature than the methyl side chains. So-called type III phase separation of aqueous PVME solution proposed by Schafer-Soenen et al. (Macromolecules 1997, 30, 410) was supported by IR and Raman spectroscopic measurements.