Abstract: The present invention relates to a process for working up reaction mixtures containing benzimidazolones of the formula (I) ##STR1## prepared by reacting o-phenylenediamines and urea in a water-insoluble solvent, where in formula (I) R.sup.1, R.sup.2 and R.sup.3 are identical or different and independently of one another are a hydrogen atom, an alkyl, alkoxy or alkylsulfonyl group having in each case 1 to 6 carbon atoms, chlorine, bromine, a substituted or unsubstituted phenyl group, a carboxylic acid, methylcarbonyl, phenylcarbonyl, methoxycarbonyl, carbamoyl, methylcarbamoyl or phenylcarbamoyl group, a hydroxyl group, cyano group or nitro group, by adding a wetting agent and a defoamer to the reaction mixture, distilling off the solvent azeotropically with the addition of water and with stirring and filtering off the benzimidazolone from the aqueous suspension arising.