The thermal behaviours of alternating styrene-maleic anhydride copolymer (St/MAn), styrene-methyl maleimide copolymer (St/MMI) and copolymers with different contents of MMI were studied by means of a simultaneous thermal analyser and quadrupole mass spectrometer system (STA/QMS). In addition, special measurements were carried out by a pyrolysis-gas chromatography-mass spectrometer (Py-GC-MS) and a differential scanning calorimeter (DSC). The glass transition of the copolymers in influenced by the degree of imidization, The decomposition behaviour of pure styrene-maleic anhydride copolymer differs from that of styrene-methyl maleimide copolymer. The favoured process of degradation of maleic anhydride-rich structures is reaction in the chains, and that of methyl maleimide-rich structures is the random chain break.