Thermal decomposition behaviour of cotton fabrics graft-copolymerized with vinyl phosphonate oligomers (Fyrol 76®) (FYR) or N-methylolacrylamide (NMA) or with combination of FYR and NMA was studied in relation to P or N contents of the treated cotton samples by Curie-point pyrolysis-gas chromatography (Py-GC) and Py-GC-mass spectrometry (Py-GC-MS). The total peak areas and most of the individual peak areas of decomposition products in Py-GC chromatograms decreased by any of three types of the treatments. The peak area of hydroxyacetaldehyde decreased most drastically to 2% of that of untreated cotton by the introduction of the lowest level of FYR. The peak area did not change further with increasing P content of the FYR-treated samples while limiting oxygen index (LOI) increased with increasing P content. In the case of NMA treatment, the peak area decreased gradually with increasing N content as well as LOI of the samples. The FYR-treated sample with the lowest P content and the NMA-treated sample with the highest N content gave the same LOI value but the peak area of the FYR-treated sample was much lower than that of the NMA-treated sample. The peak area of acrolein was found to decrease with increasing LOI, regardless of the three types of the treatments. Decomposition products formed from cellulose fractions of the samples whose peak areas increased by the treatment with FYR include furan, 2-methylfuran, and 2-furaldehyde. FYR fractions of the samples graft-copolymerized with FYR alone or combined with NMA contributed to the increase in the production of methanol. Nitrogen-containing samples yielded hydrogen cyanide, acetonitrile, and acrylonitrile. The peak areas of the latter two increased in the presence of P.