Abstract. The thermolyses in hydrogen of 3-6 vol. % of 1 ,I-dichloroethene, ( E ) -1,2-dichloroethene, trichloroethene and tetrachloroethene have been examined in a tubular flow reactor at atmospheric pressure between 828 and 1068 K. Major products are HC1, lower chlorinated ethenes, ethyne, ethene, ethane and methane. Chlorinated benzenes, major products in the straight pyrolysis of these compounds, are formed in only small amounts, the reasons for which are discussed. Dechlorination occurs in part via a chain mechanism involving H-atom addition and ips0 substitution of C1. A second route, molecular elimination of HC1, also occurs but appears to become less important for the more chlorinated substrates. At 880 K, addition of a H atom to the monochloro-substituted carbon of C,HCl, takes place ca. 8 x more rapidly than it does to the dichloro site. The hydrogen-atom concentration is somewhat greater than that calculated from equilibrium dissociation of H, and varies by a factor of ca. 4 for the various substrates. Differences in hydrogen-atom concentration are larger at lower temperatures, apparently due to the presence of relatively unreactive radicals in some systems, which lead to increased chain termination.