SynopsisThermal-induced conversion of maleic and fumaric acid anion radicals produced by y irradiation at 77 K in poly(methy1 methacrylate) (PMMA) was studied by electron spin resonance (ESR) and optical absorption spectroscopic measurements. The ESR spectra of these acid anion radicals change into two-line spectra with a line separation of ca. 10 G by thermal annealing. This spectrum is assigned to a protonated radical of each acid anion radical. Anion radicals of the solutes are relatively stable below the y transition point of PMMA and the conversion reaction takes place near this point.This means that the molecular motion of matrix molecule affects the radical conversion reaction.Specific temperatures such as the glass transition point (T,) and the secondary transition point (T,) of a polymer apparently react on the conversion of these reaction intermediates in polymer matrices.2By changing the temperature of the matrices we can study the conversion process of primary anion radicals to the secondary radicals and at the same time determine the effect of the T, of PMMA on the behavior of the intermediates, that is, the effect of molecular mobility on the conversion of the intermediates.A possible scheme for the formation of secondary radicals of maleic and fumaric acid at elevated temperature is proposed.