Polymer blends are commercially important because producers and users of polymers have increasing access to recycled materials. In addition, polymer blends are capable of providing materials which extend the useful properties beyond the range that can be obtained from single polymer equivalents. The twin problems of high flammability and coping with the hazards associated with the smoke produced during burning are only two of an increasing list of demanding specifications which are required for the commercial application of many plastics. In this paper the results of a systematic study of char formation in blends of CPVC (chlorinated polyvinyl chloride) with ABS (acrylonitrile-butadiene-styrene) containing a char-forming/smoke suppressing iron(III) compound are discussed. Flammability and smoke data are also presented and how the smoke suppressant may interact with the polymer is also proposed.