[en] Samples of insulating materials, typical of those used in large generators were pyrolysed at 700⁰ in a hydrogen atmosphere and the products analysed by combined gas chromatography-mass spectrometry. It is shown that the identification of a wide range of compounds is possible and that characteristic groupings and single compounds are produced from particular insulating materials. Monitoring the generator cooling hydrogen for these products provides early warning that specific insulating materials within the generator are starting to degrade. (Auth.)