The thermal degradation of PVC-PTMS blends has been studied over the whole composition range, using thermal volatilisation analysis and thermogravimetry. In the TVA studies, degradation products have been separated and investigated and in the TG experiments both temperature programmed and isothermal conditions have been used. It has been found that these blends are homogeneous over most of the composition range when cast as films from solution. Powder mixtures of the two polymers show no evidence of interaction during degradation, but in the case of film blends, the PVC is stabilised at low to moderate PTMS contents and destabilised at very high PTMS concentrations. There is an increase in the residue of degradation to 500-degrees-C in most of the blends. The interactions are explained in terms of reaction of chlorine chain carriers in the PVC dehydrochlorination with methylene groups in the PTMS, followed by crosslinking of macroradicals.