The effects of antimony(III) oxide (flame retardant) and basic iron(III) oxide (smoke suppressant) on the thermal stability of a tertiary blend containing ABS (acrylonitrile-butadiene-styrene), PVC (poly(vinyl chloride)) and PP (poly(propylene)) have been investigated. As found in earlier work on a two-component system of ABS and PVC, the basic oxide of iron has a marked stabilizing effect on the thermal degradation of the polymer blend; however, antimony(III) oxide has a marked destabilizing effect. Two flammability parameters, namely LOI (limiting oxygen index) and smoke production are also reported.