The hydroxymethyl groups of 2-amino-2-deoxy-D-glucose units in chitosan were selectively oxidized to carboxyl groups with NO, gas to form 6-carboxy-chitosan in aqueous 0.5 mol/L acetic acid. In optimal conditions, the 52.5% hydroxymethyl groups in chitosan could be converted into carboxyl groups. When the pH value of the solution is lower than 4.0 or more than 5.4, 6-carboxy-chitosan has common characteristics of a polycation or polyanion electrolyte, respectively, and when the pH value of the solution is 4.0-5.4, 6-carboxy-chitosan has common characteristics of an amphoteric polyelectrolyte. 6-Carboxy-chitosan has same macromolecular backbones as chitosan, and its isoelectric point (pI) value is 4.9. (C) 2004 Wiley Periodicals, Inc.