Homogeneous acetylation of chitosan was carried out with acetyl chloride in 1-allyl-3-methylimidozolium chloride (AmimCl) ionic liquid. The dissolution process of chitosan in AmimCl was observed by polarized optical microscopy. The optimal reaction conditions were achieved by screening the effect of temperature, reaction time, and acylating reagents on recovery yield of acetylated chitosan and degree of substitution, which was determined by infrared (IR) spectra analysis using baseline method. The acetylated chitosan could be separated easily by water precipitation followed by filtration. The acetylation process was further confirmed by IR spectra and scanning electron microscopy, and the crystal structure was changed after the acetylation detected by X-ray diffraction spectroscopy. Compared with traditional acetylation of chitosan, the homogeneous system, mild conditions, and simply workup make this methodology more accessible to the demand of sustainable green biomass chemistry. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013