Abstract: Process for nuclear chlorination of non-phenolic aromatic compounds, said process comprising contacting and reacting a non-phenolic aromatic compound having a net Hammett .sigma. value of about -0.1 to about 2.0 with chlorine monoxide in the presence of at least one-half an equivalent amount, based on the chlorine monoxide, of an acid having a pK.sub.a no greater than that of trichloroacetic acid, provided, however, when the net Hammett .sigma. value is about 0.7 to about 2.0, the acid has a pK.sub.a no greater than that of trifluoroacetic acid.