We have studied the crystal transition behaviors from hydrated chitosan to anhydrous chitosan by X-ray diffraction analyses. Hydrated chitosan prepared by deacetylation of crustacean -chitin was subjected to the two conversion methods, hydrothermal treatment and high-humidity treatment via chitosan/monocarboxylic acid complex. The transition by hydrothermal treatment progressed with increasing treatment temperature and time, and the rapid transition occurred above 200 degrees C. Chitosan/acetic acid complex and chitosan/formic acid complex were prepared by immersing hydrated chitosan in acid solution. The transition from chitosan/acetic acid complex to anhydrous chitosan in high relative humidity condition proceeded with increasing temperature and was complete at 80 degrees C for 1 h, whereas chitosan/formic acid complex did not convert to anhydrous chitosan under the same conditions. (c) 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1065-1069