The influence of a beta-diketone on the already observed anomalous behavior of preheated metal stearates as stabilizers for PVC has been evaluated. During normal stabilization procedure, the addition of diketones reduce the stabilization times, but the pretreatment of the mixture of metal soaps or of the individual zinc soap (then added to the calcium soap) delayed such times, recovering to some extent the reduction induced by the costabilizer. The calcium soap preheated (added along with zinc stearate to PVC), on the other hand, reduced the induction times further. These findings obtained by measurement of induction times by means of loss of weight (TGA) and HCl evolution, have been supported by FTIR and visible reflectance studies. While using preheated ZnSt(2), a second rate of dehydrochlorination was observed in the HCl evolution traces, suggesting a separate consumption of the compounds during the stabilization mechanism. In general, it can be said that preheated primary stabilizers and costabilizers have no significant interactions between them. The final result is just the addition of their separate effects.