Abstract: A urethanized polyester is claimed which is suitable as an additive for aqueous coating systems, which is obtainable by, in a first reaction stage (A), subjecting(a) from 10 to 45 mol-% of one or more diols,(b) from 5 to 50 mol-% of one or more polyols having at least 3 OH groups, and(c) from 35 to 47 mol-% of one or more aromatic, aliphatic or cycloaliphatic polycarboxylic acids and/or reactive derivatives thereof to a polycondensation reaction in a manner known per se until the reaction mixture has an acid number of from 1 to 10, and, in a reaction stage (B), reacting the resulting reaction product in a manner known per se with an organic polyisocyanate. The urethanized polyester is outstandingly suitable as an additive for aqueous coating materials in order to improve the rheological stabilization of coating materials.