In this paper, from many new examples, our approach on the preparation of chitins and chitosans with controlled physico-chemical characteristics was presented. The chitosan samples were prepared from a-chitin from crustacean shells and P-chitin from squid pens. The chitin deacetylation was carried out according to two methods using, respectively, as alkaline agent, the aqueous sodium hydroxide solution and anhydrous potassium hydroxide. The role of the source and of the process on the N-deacetylation reactions is confirmed. The effect of the addition of sodium borohydride or thiophenol within the reaction medium was studied. One of the parameters conditioning the physicochemical characteristics of chitosan being in relation with the nature and the quality of original chitin, the role of this parameter was examined and the isolation process was discussed to put in evidence its advantages compared to other processes quoted in the literature by relying on new results. (C) 2003 Elsevier Ltd. All rights reserved.