Thermogravimetric and derivative thermogravimetric analyses, and limiting oxygen index measurements of blends of epoxidized natural rubber (ENR) with rigid and plasticized poly(vinyl chloride) (PVC) were carried out. The rate of HCl elimination and the related degradation process occurring at the first step of degradation of PVC decreased with the increase of ENR content in blends. In the second step of the degradation process, however, the thermal stability showed a decline with the increase of ENR content in blends in both the rigid and plasticized PVC series. Limiting oxygen index measurements revealed that (a) PVC increased the flame resistance of ENR and (b) rigid PVC/ENR blends have higher flame resistance.