This study evaluates the thermal and oxidative stability of a specific polymer composite. A TGA/FTIR combined technique was employed to characterize this polymer/graphite fiber composite. The polymer composite sample was pyrolyzed in nitrogen, air and 2% oxygen atmosphere, respectively, in a Perkin-Elmer thermo-gravimetric analyzer. The use of a gas-cell Fourier transform IR spectrometer downstream from the TGA allowed for the identification of the evolved gas products, the thermal oxidative stability (TOS) of this composite was compared to its thermal stability; also, comparison of the TOSs in different atmospheres is presented. In addition, the effects of specimen geometry in terms of sample thickness and surface area to volume ratio (A/V) were investigated and are discussed by relating gas diffusion into the specimen to its relative stability.