Carboxymethyl chitosan (CM-chitosan) was prepared by chemical reaction with monochloroacetic acid under various conditions, and the chemical structure was analyzed by IR and NMR. The water solubility of the CM-chitosans had close relationships to the modifying conditions and the degree of carboxymethylation. The CM-chitosans, prepared at temperatures of 0-10 8C were soluble in water. But the CM-chitosan prepared between 20 and 60 8C were insoluble in the water at near-neutral pH. The water insolubility of CM-chitosans at various pHs varied with the degree of carboxymethylation. The increase in reaction temperature increased the fraction of carboxymethylation and increased the insolubility at lower pHs; the increase of the ratio of water/isopropanol in the reaction solvent decreased the fraction of carboxymethylation and increased the insolubility at higher pHs.