Products of the condensation of phenolmonosulfonic acids, dihydroxydiphenyl sulfones, urea and formaldehyde can be obtained by(A) preparing a homogeneous reaction mixture in sulfuric acid from the phenolmonosulfonic acid and dihydroxydiphenyl sulfone in the molar ratio of from 3:1 to 10:1, with the proviso that the water content of the mixture is not more than 10% of the total weight of the components, and the mixture contains from 0.3 to 1.5 moles of sulfuric acid per mole of dihydroxydiphenyl sulfone, at from 100.degree. to 180.degree. C.,(B) subsequently, at from 40.degree. to 90.degree. C., carrying out a precondensation with 1 to 2 moles of formaldehyde and 0.5 to 1.5 moles of urea per mole of phenol units present in aqueous medium,(C) partially neutralizing the reaction mixture,(D) adding to the partially neutralized reaction mixture 0 to 0.5 mole of phenol per mole of phenol units present and then 0.1 to 0.8 mole of formaldehyde per mole of phenol units then present,(E) further condensing at from 40.degree. to 90.degree. C.,(F) shifting the pH of the reaction mixture into the neutral range and(G) subsequently acidifying with a weak acid.