Effects of antimony trioxide in combination with halogen and nonhalogen based additives in a matrix of PVC/ENR miscible blends were studied for processibility, miscibility of blend components, strength properties, thermal stability, and flammability. Flammability measurements were carried out by limiting oxygen index measurement. The impairments in processibility and strength properties were within tolerable limits. Additive combinations did not alter the miscibility of blend components. Thermal stability was greatly influenced. Synergistic flame retardation of zinc borate with "Sb2O3" with and without decabromodiphenyl ether was explained by a vapor phase-condensed phase flame retardation mechanism. The antagonistic effect of phosphorus/antimony was greatly deactivated by a halogen additive and/or zinc borate.