N-Phosphonomethylation of chitosan reaction was studied and optimized using different reaction conditions. NMR spectroscopy was an important tool in this work to study this reaction and the g.-aminomethylphosphonic acid function introduced onto chitosan was unequivocally characterized by P-31 and C-13 NMR analyses. But surprisingly, whatever the reaction conditions used, N-phosphonomethylation reaction of chitosan cannot be dissociated from a side reaction: the N-methylation reaction of chitosan. A mechanism was proposed to explain the formation of this side product. (C) 2004 Elsevier Ltd. All rights reserved.