A copolymer of the monomers of the following components A to C and, if required, D, the total amount by weight of which is 100 mol %: a: from 20 to 80 mol % of at least one ethylenically unsaturated dicarboxylic acid or of an anhydride, ester and/or amide thereof as component A, b: from 1 to 79 mol % of at least one diketene of the general formula (I)  where R1 and R2 independently are each hydrogen or C1-C30-alkyl, as component B, c: from 1 to 79 mol % of at least one vinyl ester, alkyl (meth)acrylate, (meth)acrylonitrile or mixtures thereof as component C and d: from 0 to less than 0.5 mol % or, if at least 20 mol % of component C are present, from 0 to 20 mol % of at least one further olefinically unsaturated monomer as component D, is used as a flow improver in mineral oil middle distillates.