The thermal degradation of blends of PVC with polydimethylsiloxane (PDMS) has been studied by thermogravimetry (TG) over the whole composition range and by thermal volatilisation analysis (TVA) for 1:1 by weight blends. From the TG data, curves have been constructed to enable the experimental weight loss behaviour to be compared with that expected for the composition in the absence of any effects due to mixing. The results show destabilisation at low loadings of PDMS for degradations at atmospheric pressure, but for compositions with 50% or more PDMS, both polymers are stabilised, particularly the PDMS, which shows much slower weight loss than expected when 10% PVC is present. Degradation products in the TVA experiments have been separated and investigated: there are no new volatile products in the case of the blends. An interpretation of the reasons for the observed effects is presented.