Esterified cellulose ethers comprising groups of formula —C(O)—CHR′—CHR″—COOA, wherein, R′ and R″ are hydrogen or (—S)m(—R1)n-R2, wherein R1 is a hydrocarbon group having 1 to 4 carbon atoms, R2 is an optionally substituted 5- or 6-membered cyclic group, m and n each independently are 0 or 1, and A is hydrogen or a cation, with the proviso that in each group CHR′—CHR″ one of R′ and R″ is hydrogen and the other one is (—S)m(—R1)n-R2, are useful as excipients for maintaining the concentration of poorly water-soluble drugs in aqueous liquids at supersaturation levels.