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The thermal decomposition of a polyester and a polyether flexible foam in a nitrogen atmosphere has been studied by gas chromatography, mass spectrometry and elemental ultramicroanalysis. It is shown that the decomposition behaviours of the two foams are similar. At low temperatures (200 to 300C) there is a rapid and complete loss of the tolylene diisocyanate unit of each foam as a volatile smoke leaving a polyol residue. This smoke, which has been isolated as a yellow solid (common to both foams) is shown to contain all the nitrogen of the original foams. Under the conditions of this report the smoke is stable at temperatures up to 750 C. The nitrogen-containing products of low molecular weight (mainly hydrogen cyanide, acetonitrile, acrylonitrile, pyridine and benzonitrile) observed during the high temperature decompositions of the foams are shown to be derived from the decomposition of the yellow smokes. At 800C, the 5 products listed above are present. At 900C, hydrogen cyanide and benzonitrile are predominant and at 1000C, hydrogen cyanide is virtually the only product. At 1000C, approximately 70 per cent of the available nitrogen of the foams has been recovered as hydrogen cyanide.