The use of sulfuric monoesters of the formulae (I) and (II) R.sup.1 --O--(CH.sub.2 O).sub.n --SO.sub.3 .sup.- M.sup.+ (I) R.sup.1 --CO--NR.sup.2 --(CH.sub.2 O)(.sub.n+1)--SO.sub.3 .sup.- M.sup.+ (II) in which R.sup.1 is a straight-chain or branched alkyl group of 10 to 24 carbon atoms, R.sup.2 is a hydrogen atom or a C.sub.1 -C.sub.6 -alkyl group, M.sup.+ is a hydrogen ion, an alkali metal ion, 1/2 an alkaline earth metal ion, or an ammonium ion of the formula N.sup.+ R.sup.3 R.sup.4 R.sup.5 R.sup.6, where the substituents R.sup.3, R.sup.4, R.sup.5 and R.sup.6 independently of one another are each hydrogen atoms or C.sub.1 -C.sub.6 -alkyl groups, which alkyl groups can be substituted by 1 to 3 hydroxyl groups, and n is a number from 0 to 4; or of a mixture of at least two sulfuric monoesters of the formulae (I) and/or (II), for preparing surface-treated pigments from the classes of the quinacridones and dioxazines.