Thermal decomposition of hexachlorocyclopentadiene (HCCP) has been studied in inert gas and under oxidative conditions in a silica flow reactor at a residence time of 5.0 s between 690 and 923 K and 1 atm pressure. Pyrolysis was initiated by Cl bond fission to form pentachlorocyclopentadienyl radical; two such radicals then combined to undergo a series of intramolecular rearrangements and Cl fissions, producing principally octachloronaphthalene (8ClNP) and Cl-2. This process has been studied by quantum chemical calculation, and a reaction potential energy surface has been developed. The rate constant of initial Cl atom fission has been calculated by canonical variational transition state theory as k = 1.45 x 10(15) exp(-222 +/- 9 kJ mol(-1)/RT) s(-1) between 500 and 2000 K A minimal kinetic model was developed to model the decomposition and major products. Oxidative decomposition was studied in nitrogen with O-2 contents of 1, 6, 12, and 20 mol %. Increasing O-2 to 6-8% increased the rate of decomposition of HCCP and decreased the yield of 8ClNP. Above 823 K, hexachlorobenzene (HCB) and CO became major products. The oxidative reaction has also been studied quantum chemically. At high O-2 content (>similar to 10%), the rate of decomposition of HCCP declined as did yields of 8ClNP and HCB, but CO yields increased.