The present invention relates to a process for the preparation of salts of substituted or unsubstituted phthalic acids, by reacting a compound of the formula ##STR1## in which R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are identical or different and are H, F, Cl, Br, CF.sub.3, OH, an alkoxy or alkyl radical in each case having 1 to 4 carbon atoms, or a radical --NR.sup.7 R.sup.8, in which R.sup.7 and R.sup.8 are identical or different and are H, an alkyl radical having 1 to 4 carbon atoms, or a phenyl radical, R.sup.5 and R.sup.6 are identical or different and are H, a --CO-- alkyl group having 1 to 6 carbon atoms in the alkyl radical, or a benzoyl group, or R.sup.5 and R.sup.6 together form a radical of the formula ##STR2## in which R.sup.9, R.sup.10, R.sup.11 and R.sup.12 are identical or different and are H, F, Cl, Br, CF.sub.3, OH, an alkoxy or alkyl radical in each case having 1 to 4 carbon atoms, or a radical --NR.sup.7 R.sup.8, in which R.sup.7 and R.sup.8 are identical or different and are H, an alkyl radical having 1 to 4 carbon atoms, or a phenyl radical, with water, a water-soluble base, and an oxidizing agent at a temperature of -10.degree. to 150.degree. C. in the presence or absence of a water-insoluble solvent inert under the reaction conditions.