We present the results of a study of the thermal degradation of hexachloropropene in a tubular flow reactor with in-line GC-MS product analysis. Hexachloropropene was observed to be thermally fragile with initial reaction products (T < 773 K) including CCl 4 , C 2 Cl 4 , C 2 Cl 6 , and C 3 Cl 4 , (a). At higher temperatures (up to 1223 K), pronounced molecular growth was observed with reaction products including C 4 Cl 6 , C 6 Cl 6 (cy), C 6 Cl 8 , C 8 Cl 8 (cy), and C 1 2 Cl 8 (cy). Kinetic modeling of observed product yields indicated that Cl displacement of CCl 3 radicals was the dominant initiation pathway for conversion of C 3 Cl 6 into C 2 Cl 4 , CCl 4 , and C 2 Cl 6 . Four reaction submodels were considered in developing a model for the formation of C 6 Cl 6 (cy): recombination of C 3 Cl 5 , recombination of C 3 Cl 3 , pericyclic addition of C 3 Cl 4 (a), and addition reactions of C 2 and C 4 unsaturated radicals with C 2 Cl 2 . Recombination of C 3 Cl 5 radicals accounted for all of the observed yields at low temperatures (T < 873 K). At higher temperatures, C 3 Cl 3 recombination accounted for about 80% of observed yields with C 3 Cl 5 recombination accountable for the remainder. Purely C 4 radical-molecule reactions were also shown to make significant contributions to formation of octachlorostyrene (C 8 Cl 8 (cy)). Consequently, the more conventional C 2 molecular growth pathways were observed to be insignificant for this system.