Thermal transitions of poly(N-isopropylmethacrylamide) (PIPMAm) in aqueous solutions were studied in a wide temperature range from - 100 to 60degreesC. The solutions of different polymer weight fractions (c = 0.03-1) were investigated in cooling and heating scans of differential scanning calorimetry; four qualitatively different kinds of thermograms were identified in dependence of the fraction, c. The transitions are explained in a thermodynamic framework based on the phase diagram of the system. This diagram provides a complete understanding of thermal transitions in aqueous PIPMAm solutions as a function of c, in particular the existence of non-crystallizable water and the temperature induced phase transition that is due to the hydrophobic interaction in PIPMAm solutions. (C) 2004 Elsevier Ltd. All rights reserved.