4-alkoxy-2-hydroxybenzophenone-5-sulfonic acids of the general formula I ##STR1## where R.sup.1 is C.sub.1 -C.sub.20 -alkyl which is unsubstituted or substituted by halogen, cyanao, hydroxyl, C.sub.1 -C.sub.20 -alkoxy, C.sub.2 -C.sub.20 -alkoxycarbonyl, acyloxy and/or phenyl which is unsubstituted or substituted by C.sub.1 -c.sub.4 -alkyl, C.sub.1 -C.sub.4 -alkoxy and/or halogen, R.sup.2 and R.sup.3 independently of one another are each hydrogen, halogen, C.sub.1 -C.sub.12 -alkyl, C.sub.1 -C.sub.12 -alkoxy, C.sub.1 -C.sub.4 -haloalkyl, C.sub.3 -C.sub.8 -cycloalkyl, C.sub.4 -C.sub.12 -cycloalkylalkyl, cyano, hydroxyl or hydroxyethyl or are each phenoxy, C.sub.7 -C.sub.10 -phenalkyl or phenyl which is unsubstituted or substituted by C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.4 -alkoxy and/or halogen, and R.sup.4 is hydrogen or sulfo, are prepared by reacting a 4-alkoxy-2-hydroxybenzophenone of the general formula II ##STR2## with chlorosulfonic acid at from -20.degree. to +100.degree. C., by a novel and improved process in which the reaction is carried out in an aliphatic, cycloaliphatic and/or aromatic carboxylate as solvent.