The thermal degradation of an ABA type block copolymer of poly( tert-butyl aziridine) and poly(methyl methacrylate) has been studied using thermogravimetry, differential thermal analysis and thermal volatilisation analysis (TVA), with programmed heating at 10 °C/min. Volatile products have been separated by subambient TVA and characterised. The liquid fraction was subjected to gas chromatography for further separation and the components were identified by mass spectrometry. The cold ring (tar/wax) fraction was examined by IR spectrometry. The main volatile products of thermal degradation of the copolymer were identified as isobutene and aziridine, together with smaller quantities of ethylene, carbon dioxide and propene. Methyl methacrylate, methanol, α-methylstyrene, 2-phenyl-2-butene, 1- tert-butylamino-2-isopropenylaminoethane and analogous materials were the main liquid products. The cold ring fraction, comprising more than half the total evolved products, consisted of oligomers of TBA and MMA. Mechanisms have been proposed to account for the products formed.