Autocatalysis during the thermal dehydrochlorination of poly(vinyl chloride) (PVC) is shown to be a free-radical process that converts the ordinary monomer units of the polymer into chloroallylic structures that have low thermal stabilities. In the first stage of dehydrochlorination, conjugated polyene sequences are created by a nonfree-radical route. They react with HCl to give cation monoradicals and/ or excited cation diradicals. One or both of these species, or other radicals formed from them, can then abstract methylene hydrogen in order to produce new radicals that are also carbon-centered. These are converted by chlorine-atom P scission into the chloroallylic segments, which start the growth of new polyenes in the usual (nonradical) way. At 180 degreesC in solid PVC, autocatalysis was inhibited by free-radical scavengers (a hindered phenol, triphenylmethane, and metallic mercury) but greatly enhanced by an increased concentration of HCl when all-trans-beta-carotene, a model for PVC polyene sequences, was introduced simultaneously. When they were subjected to autocatalytic conditions, other model compounds gave products that apparently resulted from the abstraction of hydrogen by free-radical intermediates.