Gamma-irradiated poly(methyl methacrylate) (PMMA) with high doses 320, 480, 640 and 800 kGy were studied by EPR spectroscopy at 25 degreesC. The EPR spectra can be accounted for by the presence of two radicals: Ra and Rb. Ra has a well-known nine-line spectrum, which is attributed to the main chain scission radical. Rb has a broad single-line spectrum, which is observed only under high dose irradiation condition. The line positions of the EPR spectrum of Ra with irradiated dose of 13.6 kGy, can be simulated with the following parameters: g = 2.0036; a = 23(3H), 7.3(1H) and 13.3(1H) G; peak-peak linewidth (DeltaH(pp)): 7.3 G. However, serious lineshape discrepancy is obvious. On the other hand, an artificial spectrum, which is the appropriate sum of a five-line spectrum (a = 23 G; g = 2.0036; DeltaH(pp) = 4.6 G) and a four-line spectrum (a = 23 G; g = 2.0036; DeltaH(pp) = 8.2 G) can faithfully reproduce the experimental spectrum of Ra. The experimental EPR spectrum of PMMA under high dose gamma-irradiation can be separated into the artificial spectrum of Ra and the spectrum of Rb. Quantitative analyses of the spin concentrations indicate that [Ra] decreases while [Rb] increases with increasing irradiation dose. When annealed at 40 degreesC, the concentrations of [Ra] and [Rb] are followed a second order annihilation process and their time constants are 25 1 and 230 min, respectively. (C) 2002 Elsevier Science B.V. All rights reserved.