The dynamic behavior of water in crosslinked chitosan (cr-CS) was studied at the molecular level by proton nuclear magnetic resonance (H-1-NMR) spectroscopy and positron annihilation lifetime spectroscopy (PALS). Proton line width was used as a direct parameter of local mobility during swelling. The dynamic behavior of water in hydrogel can be described by the two following processes : physical movement along free volume cavities in network and binding with hydrophilic groups of hydrogel. The formation of bound water and the effect of water on polymer network were correlated with the relaxation of hydrogel network.