Peroxide-initiated cross-linking of high-density polyethylene (HDPE) was studied using an on-line electron spin resonance (ESR) technique. Peroxides were used to provide primary radicals upon thermal decomposition at elevated temperatures for the generation of polymer backbone radicals. Recording of the ESR spectra during the reaction process verified the free radical mechanisms involved in the modification. The determining variables for the reaction kinetics were the reaction temperature, peroxide type, and concentration level. Cross-linking via the recombination of backbone radicals was the major mechanism for high-density polyethylene modification. It was shown in the ESR spectra that some backbone radicals were trapped into the cross-linked polymer network and were still detectable after several months. The termination of backbone radicals was diffusion controlled. The rate constants were estimated using the measured radical concentration profiles. Significant changes in the signal hyperfine structure were also observed, indicating the formation of allyl radicals and the effects of melting and cross-linking on the mobility of the backbone radicals.