The decay kinetics of propagating radicals in UV-irradiated poly(methyl methacrylate) (PMMA) was followed at the irradiation temperatures in the range 300-400 K. At the highest temperatures, in the glass transition region of PMMA the radical decay patterns approached that of classical kinetics. Below the glass transition region, increase of reaction dispersivity with temperature decrease was found to be accompanied by a marked decrease, of about two orders of magnitude, of activation energy for the reaction. Both these phenomena are rationalized in terms of the stochastic model [A. Plonka and A. Paszkiewicz, J. Chem. Phys. 96, 1128 (1492)], which includes structural reorganization of the host matrix by imposing upon the static disorder model for radical reaction the additional assumption that matrix relaxation randomly reassigns the reactivity of guest species. Effective relaxation is shown to lead to the classical pattern of radical decay and the accompanying marked increase of activation energy is seen to be due to the contribution of activation energy for matrix structural relaxation.