The changes of chemical and crystal structures of the reacetylation and degraded chitosans caused by sonication, acid hydrolysis, and H2O2 oxidation were studied systematically. During the reacetylation of chitosan, acetic anhydride can result in the degradation of chitosan when its concentration exceeds a certain value (the acetic anhydride/amine ratio > 0.3). Three crystalline polymorphs; appear during the reacetylation process of chitosans that the degree of deacetylation (DD) range from 94 to 5%. The increase of acetyl group leads to a notable crystal structure transition from crystal "Form II" with constrained chain conformation to "Form I" having a more extended chain structure, and finally to a crystalline form similar to that of chitin. The DD of chitosan is not altered by sonication and acid hydrolysis, but decrease obviously because of deamination occurring in the chain scission of H2O2 oxidation. The crystallinity increases with the decreasing of molecular weight for sonication and acid hydrolysis, whereas a deeper degradation by H2O2 Will decrease the cystallinity. (c) 2007 Wiley Periodicals, Inc.