The study, by scanning and transmission electron microscopy (SEM and TEM), of the morphological aspects of thermal degradation of poly (phenylene ether) -high-impact polystyrene blends (PPE/HIPS) occurring on burning gives a detailed mechanistic insight into the process. It is seen that thermal degradation leads to a cellular structure in the material that evolves to a foamed char on burning. The fire-retardant triphenyl phosphate can modify the shape and size of the cellular structure.