Thermosensitive phase separation of aqueous solutions of the random copolymers of N-isopropylacrylamide (iPA) and N,N-diethylacrylamide (dEA) (PiPA-dEA) and of iPA and N-isopropylmethacrylamide(iPMA) (PiPA-iPMA) with different compositions has been investigated by using calorimetry, turbidimetry and infrared spectroscopy. Though the phase transition temperature (T-p) of PiPA-iPMA is a linear function of its composition, a deviation from additivity is observed for that of PiPA-dEA, that is, it has a minimum value at iPA/dEA = 1 (mol/mol). IR spectrum at the amide II mode of the copolymer suggests that part of N-H groups of iPA units form a hydrogen bond with C=O groups of dEA units at T > T-p as well as with those of the iPA units. Effects of methanol on T-p of these copolymers have also been studied. (C) 2008 Elsevier Ltd. All rights reserved.