Recent legislation introduced to limit the use of heavy metal stabilizers (cadmium based) in poly(vinyl chloride) (PVC) has necessitated the use of organic costabilizers as adjuncts to alternative main stabilizer systems (barium/zinc or calcium/zinc). It has been suggested in the literature that costabilizers substitute allylic chlorine by a C-alkylation reaction; costabilizers act as reverse catalysts for the initiation of degradation by complexing the pi-electron sites that would otherwise have an activating affect on labile chlorines; and costabilisers destroy carbonylallyl active sites by proton donation. To rationalize this debate, the focus of this paper is to elucidate the type of interactions that occur between a model compound for PVC and the novel costabiliser N-phenyl-3-acetylpyrrolidin-2,4-dione (P24D). The model compound chosen was 4-chloro-2-hexene (4C2H), which simulates the in-chain allylic chlorine impurities present in PVC and are considered the most labile defects present in the polymer. Results suggested that stabilisation involves concerted reactions involving metal complexes rather than a series of stepwise reactions. (C) 2004 Wiley Periodicals, Inc.