Peroxide-initiated reaction of poly(ethylene-co-a-butene) (EB), a kind of polyolefin elastomer (POE), was studied at elevated temperatures using an on-line electron paramagnetic resonance (EPR) spectrometer and was also evaluated by monitoring the viscosity of the reaction system on a rotational rheometer. It is clear that the reaction process experiences the free radical mechanisms. According to the EPR spectra, notable changes in signal hyperfine structure, the intensity and lifetime of the radical were recognized. Different from the case of PE and PP modified by peroxide, there is a new type of EPR signal, which is assisted to the tertiary alkyl radicals. And it is verified that the primary alkyl radicals are first produced at elevated temperatures. A rheokinetic analysis was made by EPR and rheology. The kinetic coefficients of coupling and disproportionating reactions were not fixed, but the functions of the reacting time or the system viscosity. Accordingly, the kinetics is substantially controlled by diffusion of the polymer chains and a formula reflecting this kind of effect of the viscosity on termination rate coefficients is founded. (C) 2007 Elsevier Ltd. All rights reserved.