An esterified cellulose ether comprises (i) aliphatic monovalent acyl groups or (ii) groups of the formula &mdashC(O)&mdashR&mdashCOOA wherein R is a divalent aliphatic or aromatic hydrocarbon group and A is hydrogen or a cation or (iii) a combination of aliphatic monovalent acyl groups and groups of the formula &mdashC(O)&mdashR&mdashCOOA, wherein the cellulose ether has anhydroglucose units joined by 1-4 linkages and has methyl groups, hydroxyalkyl groups, and optionally alkyl groups being different from methyl as substituents such that the esterified cellulose ether has an MS (hydroxyalkyl) of 0.05 to 1.00, and hydroxyl groups of anhydroglucose units are substituted with methyl groups such that [s23/s26&minus0.2*MS(hydroxyalkyl)] is 0.36 or less, wherein s23 is the molar fraction of anhydroglucose units wherein only the two hydroxyl groups in the 2- and 3-positions of the anhydroglucose unit are substituted with a methyl group and wherein s26 is the molar fraction of anhydroglucose units wherein only the two hydroxyl groups in the 2- and 6-positions of the anhydroglucose unit are substituted with a methyl group. The esterified cellulose ether is useful as an excipient for poorly water-soluble drugs.
