An improvement in a process for the preparation of the sulphonic acid chloride of the formula ##STR1## wherein R.sup.1, R.sup.2 and R.sup.3 are identical or different and denote hydrogen, a lower alkyl or cycloalkyl radical, halogen, nitro, aryl, aralkyl, aryl ether or a radical ##STR2## or wherein adjacent radicals R.sup.1 and R.sup.2 are linked to form a cycloaliphatic or aromatic carbocylic ring which is optionally substituted by a sulphonic acid chloride group by contacting a sulphonic acid of the formula ##STR3## wherein R.sup.1, R.sup.2 and R.sup.3 have the abovementioned meanings, with phosgene in the presence of a N,N-dialkylcarboxylic acid amide catalyst, the improvement wherein the process is carried out in the presence of a sulphonating agent.