The volatile products of thermal degradation of copolymers of methyl methacrylate and butyl acrylate covering the entire composition range have been identified. 1-Butene and carbon dioxide are the main gaseous products. Other products, methane, ethane, propane, butane, ethylene, propylene, cis and trans 2-butene, hydrogen and carbon monoxide are also present in very small amounts which tend to increase with the butyl acrylate content of the copolymer. The liquid products are butanol, methyl methacrylate, butyl acrylate and butyl methacrylate. Products have been estimated quantitatively for all the copolymers at intervals during the course of the reaction and detailed mass balance tables are drawn up. The yields of carbon dioxide and 1-butene and the molar ratio of carbon dioxide to 1-butene all increase with increasing acrylate content of the copolymer. Methyl methacrylate is the only liquid product from high methacrylate copolymers. Butyl acrylate and methacrylate begin to appear in 16,3 mole-per thousand butyl acrylate copolymer although butanol is still absent. From high acrylate copolymers butanol is the only liquid product.