The bound water fraction (X-BW) of a newly-developed pH-sensitive biodegradable chitosan-polyvinyl alcohol (PVA) hydrogel crosslinked with glutaraldehyde (GA) was investigated as a function of the chitosan/PVA molar ratio, GA concentration (C-GA), and ionization state. Differential scanning calorimetry (DSC) was used to determine the X-BW of the initial hydrogel and of the hydrogel equilibrated in pH 3 and pH 7 buffers. Changes in X-BW during swelling and shrinking of hydrogel were also investigated. In the initial state of hydrogel, X-BW increased with increasing PVA concentration (C-PVA), without being significantly affected by C-GA. In the buffer-equilibrated hydrogels, X-BW decreased with increasing CPVA and decreasing CIA. The amount of bound water based on dry mass (C-BW) was substantially higher when the hydrogel was in the ionized (swollen) state compared to its unionized counterpart. This may be due to the association of a large quantity of water molecules with -NH3+ groups of chitosan when the gel swelled in the acidic environment. 2006 Wiley Periodicals, Inc.