A water-dilutable polyester resin ABCD having a mass fraction of from 1 to 10% of units derived from unsaturated cocondensed building blocks, an olefinic double bond content of from 10 to 2 000 mmol/kg, and a sulfonic acid group content of from 20 to 300 mmol/kg, modified if desired by copolymerizing olefinically unsaturated monomers in the presence of the polyester resin, a process for preparing them, and their use in particular in one-component coating compositions in combination with amino resin curatives.