Process for decreasing the halogen content of polyhalogenated aromatics

In a process for decreasing the halogen content of polyhalogenated aromatics selected from the group consisting of benzene, naphthalene and biphenyl, each of which is polyhalogenated with one or more halogens selected from the group consisting of chlorine, bromine and iodine, and which, in addition to said halogenation, can have up to two C.sub.1 -C.sub.4 -alkyl substituents by reaction of said polyhalogenated aromatics with monohalogenated aromatics, non-halogenated aromatics, or mixtures thereof selected from the group consisting of halogenated and non-halogenated benzene, naphthalene and biphenyl which, if halogenated, are halogenated by a halogen selected from the group consisting of fluorine, chlorine, bromine and iodine, and which, whether halogenated or non-halogenated, are optionally substituted with up to two C.sub.1 -C.sub.4 -alkyl groups in the gas phase, in which halogen atoms are transferred from said polyhalogenated aromatics to said monohalogenated aromatic, non-halogenated aromatic or mixtures thereof, and at the same time hydrogen atoms are transferred from said monohalogenated aromatic, non-halogenated aromatic or mixtures thereof to the polyhalogenated aromatic, the improvement which comprises carrying out the reaction in the presence of a catalyst containing at least one element from the group of platinum metals in elemental form or in the form of compounds and with the addition of from 5 to 200% by weight of a hydrogen halide selected from the group consisting of hydrogen chloride, hydrogen bromide and hydrogen iodine.