Temperature-responsive phase separation of poly(N-(2-ethoxyethyl)acrylamide) (PEoEA, phase transition temperature, T-p = 38 degrees C) and poly(N-(2-ethoxyethyl)methacrylamide) (PEoEMA, T-p = 50 degrees C) in aqueous solutions has been investigated by infrared spectroscopy. They had a wide transition temperature region (similar to 25 degrees C), and their T-p monotonically increased with increasing concentration of added methanol, unlike re-entrant phase separation of poly(N-isopropylacrylamide) (PNiPAM). Their amide I bands consisted of two components assignable to singly and doubly hydrogen bonding C=O groups in both aqueous solutions and methanol/water mixtures. The intensity of the latter decreased during both phase separation and an increase in methanol concentration, whereas the v(C-H) bands underwent red shifts. The changes were qualitatively the same as those of PNiPAM, although PNiPAM has a higher tendency to accept double hydrogen bonds at the C=O groups. The behavior of the copolymers of N-(2-ethoxyethyl)acrylamide and N-isopropylacrylamide in methanol/water mixtures gradually approached that of PNiPAM with an increasing content of the latter, suggesting that the interaction between each individual monomer unit and solvent molecules cooperatively determines their behavior as a whole.