This study of the pyrolysis of a flexible polyurethane foam showed that the composition of the products depends on the conditions of the pyrolysis. If the volatiles are removed rapidly from the system, they will contain tolylene diisocyanate (TDI). However, under confined conditions, the TDI will be replaced by diamino toluene (DAT). These results can be explained by assuming two decomposition routes to be operative; one leading to the regeneration of the two source materials, TDI and the polyol; the other to DAT and a double-bond-terminated polyether (the polyol with its terminal hydroxy groups replaced by double bonds). The TDI route is the faster, however : if the volatile TDI is confined to the pyrolysis zone, an equilibrium will be established between the urethane group and the TDI plus polyol. Such conditions will favor the slower, though irreversible, route leading to the formation of DAT. On pyrolysis, the urea groups in the foam dissociates into TDI and DAT. These will recombine in the vapor phase to form an aerosol of polyurea. It is proposed that this aerosol is the "yellow smoke" reported in the literature.(12,27)