Thermosensitive phase separation of aqueous solutions of acrylate polymers with oligooxyethylene side groups, poly (2-(2-methoxyethoxy)ethyl acrylate) (PM2A) and poly(2-(2-methoxyethoxy)ethyl methacrylate) (PM2Ma), has been investigated by using infrared and confocal micro-Raman spectroscopy. PM2A and PM2Ma exhibited phase separation at 45 and 26 degrees C with a transition enthalpy of 21 and 36 J/g, respectively. The Raman and infrared bands for the C-H and C=O stretching modes (nu(C-H) and nu(C=O)) of the polymers exhibited red and blue shifts on phase separation, respectively, whereas the C-O stretching bands of the ether groups hardly shifted. The nu(C-H) and nu(C=O) bands also shifted with an increase in polymer concentration. Though free carbonyl groups exist in both of the polymers even at low polymer concentrations and below the phase separation temperatures, the average number of H-bonds is lower in PM2Ma than in PM2A probably because of the steric effects of the alpha-methyl groups. Raman microscopic observation showed that polymer-rich domains appeared above the phase separation temperature and the polymer concentration reached 52%.