The chemical reactions occurring during the intumescent process taking place in the combustion of the poly(2,6-dimethyl-1,4-phenylene ether)-high-impact polystyrene blends (PPE-HIPS) are studied in detail through the chemical characterization of the burnt and original material by infrared, pyrolysis-gas chromatography-mass spectrometry, and direct insertion probe spectrometry. Evidence is given of thermal rearrangement in the blend of the polyether PPE chains to polybenzylic structures occurring in the heating conditions of pyrolysis or combustion, as previously shown, to take place in thermal degradation of PPE. The rearranged chain segments are shown to give a larger contribution to the intumescent char, while volatile blowing products are mostly formed by polystyrene and polybutadiene components. From PPE-HIPS blends, the volatilization of the fire-retardant triphenyl phosphate (TPP), which when heated alone volatilizes at a temperature below that of PPE-HIPS degradation, is delayed probably by hydrogen bonding with PPE. This allows TPP to play the typical flame inhibition role of volatile phosphorus compounds. Moreover, it is found that TPP favors the PPE rearrangement and henceforth increases the char yield of the burning blend, which is a typical condensed phase fire-retardant action.