The introduction of a conjugated vinyl group to chitooligosaccharide (COS) has been easily accomplished by the coupling of glycidyl methacrylate (GMA) to COS in aqueous solution without an additional catalyst. Depolymerization of chitosan was carried out by deaminative cleavage using sodium nitrite. The average degree of polymerization of COS could be controlled by varying the molar ratio of sodium nitrite and the amino group of chitosan. The degree of substitution (percentage), as determined by H-1 NMR, of GMA to COS increased up to 60% at a reaction time of 48 h. The structure of products (COS-GMAs) were identified by Fourier transform infrared spectroscopy, H-1 NMR, and C-13 NMR. The resulting COS-GMAs were readily soluble in neutral water, like the original COS, and exhibited an excellent antimicrobial activity. The reactivity of COS-GMA was investigated by the reaction with poly(vinyl alcohol) (PVA). The reaction products were found to have a lower crystallinity than WA because of the introduction of the COS-GMA units. (C) 2001 John Wiley & Sons, Inc.