Ultra-high purity hexahalogencyclopentadienes are produced by feeding a crude halogenocyclopentadiene (pref. hexachloro-cyclopentadiene) into a first fractionating distillation zone where all halogenobutadienes are sepd. as a head fraction; and passing the bottom fraction into a second fractionating distillation zone where ultra-high purity halogencyclopentadiene is sepd., as a head fraction, whereas all halogenocyclopentene impurities remain in the bottom fraction. Hexachlorocyclophentadiene is used in the prodn. of Diels-Alder adducts (e.g. 'chlorendic anhydride' with maleic acid or fire-retarding adducts with furan. Hexachlorocyclopentadiene used in these and similar process should be as pure as possible because chlorohydrocarbon impurities have unpleasant smells, may irritate human skin, eyes and respiratory system, and cause corrosion of metal apparatus parts. The present process ensures through purification (less than 0.5 wt. %, usually less than 0.1 wt.% of halogenohydrocarbon impurities) at a relatively low cost.