The increase of the stability of chitosan (CS) in the solution is extremely important for the application of CS in various fields. In this study, we describe that the hydrogelation of CS can solve this problem. CS hydrogel was readily prepared by addition of a sodium hydroxide solution to a CS acetic acid solution. The CS hydrogel was stable in the wet state and could maintain its molecular weight for a long period of time at room temperature. The obtained CS hydrogel could be readily dissolved with addition of acids, such as acetic acid and other organic acids, as determined by measuring the pH and transmittance of the solutions. Dissolution of the CS hydrogel indicated that the amount of carboxylate ion per amino group of CS was only 0.5 molar equivalents. Moreover, the binding state between the acetic acid and amino groups of CS in an aqueous solution is discussed using results from H-1 NMR spectroscopy. The methyl signals attributed to free acetic acid, acetic acid in an ionic bond, and the acetamido group of the N-acetylglucosamine residue were clearly observed, and the strength of each peak changed quantitatively with the addition of acetic acid. (C) 2017 Elsevier B.V. All rights reserved.