Biodegradable hydrogels are finding increasing interest in the academic and industrial field due to their high swelling capacity and the potential for many novel applications enabled by their biodegradability. The monitoring of the hydrogel cross-linking process is a crucial step for predicting hydrogel performances in terms of degree of swelling and viscoelastic properties. In this work, the chemical cross-linking of cellulose based hydrogels has been monitored during synthesis in water by means of ultrasonic wave propagation and low frequency dynamic mechanical analysis (DMA). The effect of the cross-linker concentration on the hydrogel acoustic behaviour has been also analysed and correlated with the different elastic response developed by the macromolecular hydrogel. The results demonstrate the reliability of the ultrasonic wave propagation in the following network formation process of a superabsorbent hydrogel, being capable of following the limited changes in the physical properties of the reacting solution. (C) 2005 Elsevier Ltd. All rights reserved.