The detailed chemical reactions involved in the thermal ageing (at 120 degrees C) of a typical aerospace epoxy composite have been Studied by the mid-Fourier transform infrared spectroscopy (FTIR) spectral characterization of selected model compounds. The FTIR spectra indicate that at this particular temperature, the major reaction in the resin structure is probably an oxidation of a CH, group adjacent to the nitrogen atom of the tetraglycidyldiaminodiphenyl methane unit of the molecular structure, particularly where it is attached to one isomer of the diaminodiphenylsulfone hardener unit. The FTIR changes indicate that the major product is an amide group but other minor changes are also detailed and differences in the chemical changes seen at other ageing temperatures highlighted. (c) 2006 Wiley Periodicals, Inc.