Process for the preparation of thioindigo compounds of the formula ##STR1## in which A and B designate identical or different optionally substituted benzene rings or benzene rings onto which further carbocyclic and heterocyclic rings are fused,characterized in that compounds of the formula ##STR2## in which A and B have the meaning indicated above andX designates halogen, such as bromine and, preferably, chlorine,are reacted in a liquid mixture of an aluminium halide, an alkali metal halide and SO.sub.2 and the resulting intermediate product is then oxidized in a manner which is in itself known, if appropriate after first being isolated, and the mixture is worked up to give the compounds of the formula (I).