Because of their high barrier to the transport of oxygen and flavor and aroma molecules, vinylidene chloride polymers occupy a position of prominence in the polymer plastics packaging industry. However, at processing temperatures these polymers tend to undergo degradative dehydrochlorination. The dehydrochlorination reaction is a typical chain process which is initiated at a defect site (allylic dichloromethylene unit), present in the polymer as a consequence of random dehydrohalogenation during polymerization, drying, etc. This initial dehydrohalogenation is thought to be catalyzed by metal ions formed by interaction of the polymer with equipment walls or present by other means. To test this, a series of blends of metal halides (5 wt%) with a vinylidene chloride/methyl acrylate (5 mol%) copolymer has been examined by thermogravimetry in nitrogen. All the polymer/metal halide blends were more prone to degradation than the untreated polymer. The effectiveness of the metal halide in promoting the degradative dehydrochlorination correlates well with the Lewis acidity/Pearson hardness of the metal ion.