The effect of alkyl group substituents on the degradation kinetics of poly(alkyl methacrylates) - namely, poly-(n-butyl methacrylate) (PBMA), poly(isopropyl methacrylate) (PPMA), poly(ethyl methacrylate) (PEMA), and poly(methyl methacrylate) (PMMA)-under supercritical and subcritical conditions was studied at various temperatures (250-375 °C). The molecular-weight distributions were measured via gel permeation chromatography, and continuous distribution kinetic models were used to determine the degradation rate coefficients. The degradation rate coefficients under supercritical conditions were determined to be significantly higher than that those under subcritical conditions. The degradation rate of poly(alkyl methacrylates) increased as the number of carbon atoms of the alkyl substituents increased and, thus, followed the order PBMA > PPMA > PEMA > PMMA. The activation energy decreased with chain length under both subcritical and supercritical conditions. The degradation of PBMA was investigated at various pressures (20-85 bar) at a temperature of 325 °C, and the effect of pressure on the degradation rate coefficient was modeled using transition state theory. The degradation of PBMA was also investigated in different solvents at 325 °C and 60 bar. The supercritical state and the density of the solvent were determined to be important factors that influence the degradation kinetics. © 2007 American Chemical Society.