Chitin was converted into diacetylchitin and also into various kinds of acetylchitins. A fully acetylated chitin was prepared by the acetylation of chitin in acetic anhydride—methanesulfonic acid mixture at 0°C overnight, or in acetic anhydride—perchloric acid mixture at 0°C for 3 h. Variously acetylated chitins were prepared by similar procedures applying a calculated amount of acetic anhydride in glacial acetic acid. The reaction of alkaline chitin with acetic anhydride gave 0.3 acetylchitin (0.3 mol of –OH groups are acetylated per one N-acetylglucosamine residue), and heating this with anhydrous sodium acetate in acetic anhydride produced ca 1.1 acetylchitin. However, it was difficult to further improve the degree of acetylation by this alkaline method. The acetylation of chitin resulted in an increase of solubility in formic acid, and diacetylchitin was easily soluble even in 85-% aqueous formic acid. The analytical data showed that the chitin residue exists as a stable hemihydrate unless it is completely acetylated. Infrared absorption spectra for chitin and acetylchitins were also investigated.