The thermal decomposition of the epoxy resin system, MY720 (70 wt % tetraglycidyl diaminodiphenyl methane), cured with Eporal (diamino diphenyl sulfone), was investigated at 125–215°C. The primary decomposition product was propenal. Several other aldehydes were observed; however, they are formed during cure and are not thermal decomposition products. The apparent activation energy for the production of propenal was 67 kJ/mol (16 kcal/mol). Possible mechanisms for the source of this compound are discussed.