A quantitative study on the formation of carbonaceous char at 350, 500, 650 and 800 degrees C in unplasticized poly(vinyl chloride), acrylonitrile-butadiene-styrene and especially in blends of the two polymers has been carried out. Incorporating iron compounds at varying levels into the polymer blends dramatically increases char formation even at 800 degrees C. Flammability and smoke data are also presented and the relationship between char formation and levels of iron present are discussed. Crosslinking/char-forming schemes are proposed in an attempt to explain the action of iron compounds in these polymer formulations.