Second and third heterogeneous deacetylations in a multistep process under argon atmosphere of alpha- and beta-chitins in the presence of 50% (w/v) NaOH, for temperatures ranging from 80 to 110 degreesC, were comparatively studied in order to optimize the multistep process of deacetylation. Alone, with the successive reactions, we observed important changes of chemical behaviour with the crystalline state related to alpha- and beta-chitins, amorphous and partially deacetylated chitin, and chitosan. Thanks to the full reacetylation of all the deacetylated samples, we succeeded in estimating the oxidoreductive alkaline degradation occurring during deacetylation, whatever the degree of acetylation (DA) of the copolymer. It clearly appeared that the crystalline state of the samples was the key parameter on which depended the rate constants of both alkaline hydrolysis and deacetylation and, consequently, the activation energy E-a and the preexponential factor A. We may now propose optimal conditions allowing the production of well-defined chitosans with low DAs and higher molecular weights than those usually reported in the literature.