Thermal analysis demonstrates that the thermal degradation of polymethacrylamide (PMAM) occurs in two well defined steps. The only volatile products formed in the first step, below 340, are ammonia and water while imide replaces amide absorption in the infra-red spectrum of the residue. Above 340° the major product consists of chain fragments (approximately 50%) in which a high proportion of the amide groups have been converted to cyclic imides. Copolymers with methyl methacrylate (MMA) comprising more than 35% methacrylamide (MAM) also degrade in two similar steps but the overall behaviour becomes progressively more like that of polymethylmethacrylate as the MAM content is decreased below 10%. In the first stage of the reaction in the copolymer, MMA and methanol are important products in addition to ammonia and water. Chain fragments remain the major product in the second stage but a number of minor, but very significant, products are also formed. All these products and the structural features of the chain fragments and residue have been accounted for mechanistically.