A tricarboxylic acid ester of cellulose or starch is made by reacting cellulose, starch or a cellulose derivative having free and esterifiable hydroxyl groups with a tricarboxylic acid anhydride in a solvent comprising a fatty acid having 1-4 carbon atoms and in the presence of a salt, soluble in the solvent, and having an alkaline reaction in aqueous solution, as catalyst. Specified starting materials include ethyl cellulose, especially containing 45%-48.5% ethoxyl, cellulose acetate, especially the diacetate or containing 32%-39.5% acetyl, cellulose acetate-propionate, cellulose acetate-butyrate, or cellulose propionate, wood pulp, and starch. The tricarboxylic anhydride may be trimellitic anhydride, the fatty acid may be anhydrous acetic acid, propionic acid or butyric acid, and the salt may be a salt of a fatty acid having 1-4 carbon atoms, especially an acetate of an alkali metal, alkaline earth metal or organic amine. Sodium, potassium, calcium and magnesium acetates, monomethylamine acetate and pyridine acetate are specified. One part by weight of trimellitic anhydride may be reacted with one part by weight of the cellulose or cellulose derivative, and from 1 1/2 -3 parts by weight of the solvent may be used per part by weight of cellulose or cellulose derivative and trimellitic anhydride taken together. The amount of catalyst used may be from 0.05 parts to 1 part, e.g. 0.6 parts, by weight, per part by weight of cellulose or cellulose derivative, and the reaction may be continued for from 2-3 hours at about 100 DEG C. The period and conditions of reaction may be such that the ester formed contains, on average, at least 0.2 trimellityl groups per glucose unit, or contains at least 16% by weight trimellityl. Such products are water-insoluble but soluble in aqueous alkali.  Alternatively, the period and conditions of reaction may be such that the ester formed contains on average, less than 0.2 trimellityl groups per glucose unit or contains less than 16% by weight trimellityl. Such products are insoluble in both water and aqueous alkali, but may be dissolved in organic solvents. In examples, the product is separated by dilution of the reaction mixture with acetic acid, pouring the dilute mixture into water acidified with dilute hydrochloric acid, washing and drying.ALSO:Alkali-soluble trimellitic acid esters of cellulose are used as enteric coatings for medical pills and capsules.  Such esters are made by reacting cellulose, starch or a cellulose derivative having free and esterifiable hydroxyl groups with a tricarboxylic acid anhydride in a solvent comprising a fatty acid having 1-4 carbon atoms and in the presence of a salt, soluble in the solvent, and having an alkaline reaction in aqueous solution, as catalyst. Specified starting materials include ethyl cellulose, especially containing 45-48.5%, cellulose acetate, especially the diacetate or containing 32-39.5% acetyl, cellulose acetate-propionate, cellulose acetate-butyrate, or cellulose propionate, wood pulp, and starch. The tricarboxylic anhydride may be trimellitic anhydride, the fatty acid may be anhydrous acetic acid, propionic acid or butyric acid, and the salt may be a salt of a fatty acid having 1-4 carbon atoms, especially an acetate of an alkali metal, alkaline earth metal or organic amine. Sodium, potassium, calcium and magnesium acetates, monomethylamine acetate and pyridine acetate are specified.  The period and conditions of reaction may be such that the ester formed contains, on average, at least 0.2 trimellityl groups per glucose unit, or contains at least 16% by weight trimellityl.  Such products are water-insoluble but soluble in aqueous alkali.