A process for preparing an ester of a cellulose ether comprises the steps of reacting a cellulose ether with a dicarboxylic acid anhydride or with a combination of a dicarboxylic acid anhydride and an aliphatic monocarboxylic acid anhydride wherein the esterification is conducted i) in the absence of an esterification catalyst or in the presence of no more than 0.1 mole of an esterification catalyst per mole of anhydroglucose units of cellulose ether and ii) in the presence of an aliphatic carboxylic acid wherein the molar ratio [aliphatic carboxylic acid/anhydroglucose units of cellulose ether] is up to 12 /1.
