Fluorescence spectroscopy has shown that the limited sequence length exhibited during the thermal degradation of poly(vinyl chloride) (PVC) was the result of an HCl readdition/splitting off equilibrium, especially in unstabilized PVC formulations. In the case of stabilized PVC, the type and mechanistic action of the stabilisers seemed to play an important role in HCl re-addition. The possibility of a synergistic effect taking place between costabilisers for PVC, namely N-phenyl-3-acetyl pyrrolidine-2,4dione (P24D) and its derivative N-isopropyl-n-phenyl-3-(1-iminoethyl) pyrrolidine-2,4-dione (P24D-d) has also been investigated by means of statistical design, degrading the samples at 170degreesC in air and measuring the induction time (T-i) of the degraded polymer. The results reveal that the stabilising efficiency is not only affected by the presence of the metal soap, but also by the nature of the substituents in the P24D derivatives. It may be concluded that P24D-d on degradation operates as a "reservoir" for P24D, extending the Ti to higher values compared to their individual performances and giving rise to a synergistic combination. The ability of the costabiliser to form a keto-enol tautomerism as well as its ability to interact with the PVC and the metal soap play a crucial role in controlling their stabilising effectiveness in the polymer. (C) 2004 Wiley Periodicals, Inc.