The different states of water in a series of SPPESK membranes, including sulfonated poly (phthalazinone ether sulfone ketone) (SPPESK) membrane, cross-linked (XL) SPPESK membrane and SPPESK/polyacrylic acid (PAA) semi-interpenetrating polymer network (sIPN) membrane were investigated by low temperature differential scanning calorimetry (DSC) measurements. Only one melting peak was observed for each DSC curve, therefore the free water cannot be distinguished from freezing bound water in the experimental condition. The melting temperature of the freezable water was found to decrease when the content of the freezable water reduced. The content of non-freezing bound water decreased with the increase in total water uptake in each type of the membranes. It indicates that the total water uptake cannot exactly reveal the ability of tight water retention of the membranes. As compared with SPPESK/PAA membrane, SPPESK and XL-SPPESK membranes presented a higher ability of water retention because of higher content of non-freezing bound water measured. A schematic structural model is given to explain the interrelation of distribution of ionic groups (hydrophilic groups) and structure of polymers to the content of non-freezing bound water. (C) 2009 Elsevier B.V. All rights reserved.