The thermal decomposition behaviour of the mixed epoxy resin, tetraglycidyl 4,4′-diaminodiphenylmethane (TGDDM) and diglycidylether of bisphenol A (DGEBA), in a 1:1 ratio, cured with 3,3′,5,5′-tetraethyl-4,4′-diaminodiphenylmethane (M-DEA) or bis( m-aminophenyl)methylphosphine oxide (BAMPO) or with mixtures of the hardeners was studied. On heating, the resins rapidly decompose above 290 °C in an exothermic process involving dehydration, bond scission and chain fragment volatilisation. However, as the temperature increases, unsaturations created in the bond scission process polymerise to give a residue which, above 400 °C, undergoes aromatisation to form a thermostable ‘char’. In the presence of the phosphorous-containing hardener (BAMPO), the volatilisation is reduced and aromatisation is accelerated which leads to a larger amount of stable ‘char’. This is suggested to be a reason for the fire-retardant action of BAMPO which, however, at high BAMPO content is overwhelmed by flame quenching by volatilising phosphorus-containing moieties from BAMPO decomposition.