The hydrogels composed of chitosan and eugenol were prepared to enhance and sustain antioxidant activities. The vinyl groups of eugenol monomer were directly grafted on the amino groups of chitosan, using ceric ammonium nitrate. The graft of eugenol onto chitosan was confirmed by using Fourier-transform infrared and proton nuclear magnetic resonance spectroscopies. Results from the swelling behavior, thermal stability, and wide-angle X-ray diffraction revealed that the equilibrium water content decreased with increase of graft yields, because of the hydrophobicity of eugenol, although the introduction of eugenol as a side chain disturbed the ordered arrangement of chitosan's crystalline structure. The eugenol-grafted chitosan hydrogels showed lower pH sensitivity in comparison with chitosan alone, because the amino groups, which were pH sensitive, of chitosan were grafted with eugenol. The scavenging activity of the tested hydrogels increased with graft yield of eugenol, because phenolic groups in the eugenol could play a major role as potent free-radical terminators, in the results of improved antioxidant activity in eugenol-grafted chitosan hydrogel in comparison with chitosan alone. (c) 2006 Wiley Periodicals, Inc.