Some new attempts to demonstrate the superiority of the GTTG(-) conformation of isotactic triads relative to other labile structures containing allylic or tertiary chlorine in inducing the thermal degradation of poly(vinyl chloride) (PVC) are presented. They concern : (i) the degradation of polymers after nucleophilic substitution in two types of solvent involving the removal of the GTTG(-) arrangement in a quite different way; (ii) the degradation of PVC blends with a few additives which, as proved by Fourier transform infrared (FTi.r.) spectroscopy, are able to interact specifically with the mmr structure adopting GTTG(-)TT conformation at the end of isotactic sequences; (iii) the degradation of PVC after the disappearance of GTTG(-) conformations as a result of the GTTG(-)TT double right arrow GTGTTT irreversible conformational change provoked by stretching of films at temperatures near the glass transition; and (iv) the degradation of PVC after FTi.r. checked changes in content of GTTG(-) conformation, induced by annealing at temperatures between 70 and 120 degrees C. The correlations so obtained give evidence of the PVC thermal-instability-determining role of the GTTG(-) isotactic triad conformation.