To replace zinc in PVC stabilized formulations and generate a Heavy Metal Free System (HMFS), different costabilizers (i.e., 6-amino-1,3 dimethyluracil (Uracil), sodium tetraborate (Borax), sodium phenyl phosphinate (SPP), and NPPD (N-phenyl-3-acetylpyrrolidin-2,4-dione), and their combinations) were tested. HCl evolution showed the outstanding performance of SPP, which was overshadowed by its poor color stability. To optimize the levels of costabilizers in the formulation, experimental design was applied to two different formulations. For a formulation containing Uracil + SPP + NPPD, the induction time (Ti) could reach up to 50 min. Moreover, an induction time of more than 80 min could be obtained with the combination of the costabilizers Borax + SPP + NPPD. The dynamic stability in terms of color measurements showed that excellent color control was achieved by the addition of 2phr of Borax into the formulation, with even better results than the mixture of the three costabilizers at their highest levels (Borax + SPP + NPPD). For a formulation containing Uracil + SPP + NPPD, the best results were again obtained by the combination of the highest levels of the three costabilizers. The mechanistic action of SPP with a model compound (4C2H) was studied by H-1 and P-31-FTnmr. The reaction was obscured by the fact that SPP underwent hydrolysis in aqueous conditions. However, the color measurements showed that because the initial color was poor, it is unlikely that SPP is. acting as a peroxide decomposer in the conventional sense. In addition, the great extension in the Ti means that SPP Must be inhibiting chain propagation and it may do this by stabilizing the carbocation generated during the dehydrochlorination process, along with its acid scavenging activity. Thus, when the costabilizer is totally consumed, the rate of free HCl evolution is very high, as the results confirm. The fact that phenyl phosphinic acid (PPA), the acid form, did not stabilize PVC seems to be in agreement with the above reasoning. (C) 2005 Wiley Periodicals, Inc.