Self assembled monolayer (SAM) films of carboxymethyl chitosan (CM-CHI) were prepared by immersing mica in CM-CHI aqueous solution at pH = 1.7. The obtained films were further rinsed in baths of water at different pH. The pH induced morphological change of carboximethyl chitosan SAM films was investigated by atomic force microscope using tapping mode. The results showed that the morphology of the films was remarkably affected by the pH of rinse bath. Three obvious morphological changes were discussed and explained by the competition between electrostatic interaction and hydrogen bond. We draw attention to the "Crater'' structures in the films and find that the formation and evolution of "Crater'' structures have a close relationship with the pH of rinse bath. The formation mechanism of the other two special structures was also discussed. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 993-998, 2011