The thermal degradation behavior of blends of polyvinyl chloride) with polyacrylonitrile, poly(n-butyl methacrylate), poly-acrylamide, poly(N-butyl methacrylamide) and poly(methyl acrylate) is discussed, and the results for these blends and five other binary polyblends containing PVC previously studied are compared. The various types of interactions which can occur in these heterogeneous systems are considered, and the mechanisms of degradation are compared with those of each polymer when degraded alone. The most important interactive effects result when a small reactive species produced in the degradation of one of the two polymers in the blend diffuses into the domains of the other polymer in the two-phase system and reacts with that polymer.