Effects of four flame retardant additives; antimony trioxide, zinc borate, chlorinated paraffin wax and decabromodiphenyl ether in a PVC-epoxidised natural rubber miscible composition in the presence of a phthalate plasticizer and a phosphate plasticizer were studied. Mixing was done in a Brabender plasticorder at 180 degrees C. Incorporation of additives did not affect the miscibility of the blend components, and the strenght property retained between 83 percent and 91 percent. The processibility of the blend was improved due to chlorinated paraffin wax and slightly impaired due to zinc borate. Antimony trioxide and decabromodiphenyl ether had little influence on the processibility. The flame retardant additives were more effective in a phthalate plasticizer medium than in a phosphate plasticizer medium. The thermal stability of components was mutually influenced by the additive and the matrix. The vapor phase inhibiting action of antimony trioxide in PVC compositions was more pronounced in a phthalate medium than in a phosphate plasticizer medium. Phosphate plasticizer in general induced an condensed phase mechanism in the presence of flame retardant additive.