Differential scanning calorimetry (DSC), positron annihilation lifetime spectroscopy (PALS), and NMR technique were used to explore the origin of unfrozen bound water in gelatin samples with various water contents. The results reveal that the content of unfrozen bound water (W-b) increases upon increasing water content up to 3.75 g water/g gelatin, above which, W-b declines. Moreover at 50 g water/g gelatin, no unfrozen bound water was detected. It is proposed that the 'nanocavity' in polymers is an important reason for the formation of unfrozen water; hydrogen-bonding is not the 'exclusive' factor influencing water crystallization, and hydrogen-bond bound water is only one of the different physical states of water in polymers. (C) 2001 Elsevier Science Ltd. All rights reserved.