Hexafluorobutadiene has been shown to be a highly reactive compound resembling in general butadiene rather than the relatively inert hexachlorobutadiene.The thermal reactions of hexafluorobutadiene were investigated over the temperature range 150 to 180°and hexafluorocyclobutene, dimers, which were formed in largest amount, and trimers isolated as the principal products.Hexafluorobutadiene dimer and trimer fractions were shown to be mixtures of isomers. On heating, the dimer was partially converted into a remarkable lower-boiling saturated and highly stable isomer for which a fused tricyclic structure was proposed. Hexafluorobutadiene trimer polymerized slowly on heating. Both dimer and trimer were partially depolymerized by passing through a hot tube.11 haca, X. Y.