The preparation of carboxymethyl (CM)Cassia tora gum (CTG) with monochloroacetic acid and sodium hydroxide was investigated under different reaction conditions. The carboxymethylation reaction was studied with respect to the degree of substitution of CM-CTG. The variables studied were the concentration of reactants, methanol-water ratio (solvent), reaction time, and reaction temperature. Carboxymethylation of CTG increased its solubility in cold water and the stability of its paste to microorganisms. Regardless of the degree of substitution of these products, their aqueous solutions were characterized by non-Newtonian pseudoplastic behavior. (C) 2003 Wiley Periodicals, Inc.