In this study we applied temperature jump/Fourier transform infrared spectroscopy, once called fast thermolysis/FT-IR spectroscopy, to quantify the thermal decomposition gases from two kinds of fire-retardant polymers. In the case of nylon-6, the addition of melamine-cyanurate accelerated the reduction of molecular weight, which caused the function of molten polymer drip away from the ignited flame. In the case of a fire-retardant PET, the evolution of a large amount of carbon dioxide (incombustible gas) dominated in the extinguishing stage.