The interaction of chitosan and chitin with monovalent and divalent late transition metal ions was studied by means of density functional theory. The calculations were performed at the B3LYP/6-31+G** level of theory using glucosamine and N-acetylglucosamine monomers as models of chitosan and chitin, respectively, in the absence and in the presence of a few water molecules. The calculations suggest that N-acetylglucosamine is more acidic than glucosamine and that the most stable metal complexes with each of these two molecules have similar stabilities. In the case of the interaction of these two molecules with monovalent cations, the most stable complexes are those with Ni(I). In the case of the divalent cations, complexes with Cu(II) are more favourable, which is in good agreement with the available experimental data. (C) 2013 Elsevier Ltd. All rights reserved.