Like poly(methyl methacrylate), poly(n-butyl methacrylate) yields monomer as the predominating volatile product of thermal degradation at 250°C. Unlike poly(methyl methacrylate) which gives quantitative yields of monomer, however, poly(n-butyl methacrylate) only degrades to monomer to an extent of approximately 40%. On prolonged heating the residue becomes progressively more stable. The clue to this behavior is given by the fact that traces of 1-butene are detectible among the volatile products and anhydride structures in the residue. Thus depolymerization and ester decomposition to acid proceed simultaneously during the thermal degradation of this polymer. Competition between these two processes is a general property of methacrylates, with poly(methyl methacrylate) and poly(tert-butyl methacrylate) representing the two extremes of behavior. This interference by ester decomposition prevents a quantitative comparison of the thermal depolymerization of the poly(alkyl methacrylate) series but it has been possible to show that as in poly(methyl methacrylate) the depolymerization process in poly(n-butyl methacrylate) is initiated thermally at the chain ends. When copolymers of n-butyl methacrylate and methacrylic acid are heated to 250°C., conversion of acid to anhydride is rapid and complete. Yet the polymer degrades to the same extent as ester homopolymer. On the other hand, preheating the copolymer at 170°C. has an inhibiting effect on subsequent degradation at 250°C. Rapid heating to 250°C. apparently results in intermolecular anhydrides which can be liberated in a depolymerization reaction. Preheating at 170°C. allows acid units to migrate along the polymer chain so that intramolecular anhydride structures can be formed by elimination of water between adjacent acid units. These are the true inhibitors of depolymerization. Unlike the thermal reaction, the photochemically initiated depolymerization to monomer at 170°C. is quantitative. That also appears to be a general property of poly-(alkyl methacrylates) and even applies to poly(tert-butyl methacrylate) which thermally undergoes almost quantitative ester decomposition. There is no self-inhibition as in the thermal reaction. As in the thermal degradation, however, the presence of methacrylic acid units in the polymer has no effect, but preheating depresses the rate of depolymerization. Certain features of these reaction lead to the conclusion that ester decomposition in poly(n-butyl methacrylate), unlike that in poly(tert-butyl methacrylate) is a radical process occuring in direct competition with depolymerization. This is supported by the observation that during the polymerization of n-butyl methacrylate at 30°C., isobutene is produced and acid units appear in the polymer. A unified picture of ester decomposition during polymerization and depolymerization is presented in terms of such a radical mechanism. Mechanisms are also suggested to explain selfinhibition in the thermal reaction and the inhibiting effect of preheating acid/ester copolymers on both their thermal and photochemical depolymerizations.