The thermal degradation of poly(ether imide) (PEI) was studied through a combination of thermogravimetric analysis and stepwise pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) techniques with consecutive heating of the samples at fixed temperature intervals to achieve narrow temperature pyrolysis conditions. The individual mass chromatograms of various pyrolysates were correlated with pyrolysis temperatures to determine the pyrolysis mechanism. The major mechanisms were two-stage pyrolysis, involving main-chain random scission, and carbonization. In the first stage, the scission of hydrolyzed imide groups, ether groups, and isopropylidene groups produced CO+CO2 and phenol as the major products and was accompanied by chain transfer of carbonization to form partially carbonized solid residue. In the second pyrolysis stage, the decomposition of the partially carbonized solid residue and remaining imide groups formed CO+CO2 as the major product along with benzene and a small amount of benzonitrile. The yield of CO+CO2 was the largest fraction in the total ion chromatogram of the evolved gas mixtures. Hence, the thermal stability of the imide group was identical to the maximum thermogravimetry loss rates in the two-stage pyrolysis regions. Afterward, carbonization dominated the decomposition of the solid residue at high temperatures.