Abstract The action of hexachloropropene on trichloroethene gives rise to a condensation with one or two molecules of the latter. With one molecule of trichloroethene, a nonachloropentene is formed, which reacts with sulphuric acid giving a mixture of hexachlorobutene carboxyle acids. The latter is transformed into a mixture of pentachlorobutadiene carboxylic acids by the action of alcoholic potash solution.