Kinetic analyses based on the resonance delocalization energies of polyenes and polyenyl radicals are used to rule out the free-radical mechanisms of Arlman and of Winkler and Stromberg for the growth of conjugated polyenes during the thermolysis of poly(vinyl chloride) under nonoxidizing conditions. Possible sources of radicals during the thermolysis are described, and the radicals that cause the greatest reduction in thermal stability are identified as those that result from the reaction of polyenes with HCl. The way in which these radicals accelerate dehydrochlorination is discussed. J. VINYL ADDIT. TECHNOL., 2012. © 2012 Society of Plastics Engineers