The hybrid CBS-QB3 electronic structure approach is used to calculate the reaction energetics for decomposition reactions of radicals formed from UV radiation of the polymer poly(methyl methacrylate) (PMMA). Relative to the photon created radical species, the decarboxylation reaction to form CO2 is exothermic whereas the reaction to form CO is endothermic. For degradation of the polymer in low-Earth orbit (LEO) conditions, the synergy of the reaction with O(P-3) atoms is considered. The addition of O atoms to all of the radicals is exothermic, leading in many cases to the formation of a stable molecule and another radical species. (c) 2005 Elsevier B.V. All rights reserved.