AbstractAn ESR method was used to investigate the formation of free radicals during rubber mastication and filling of rubber with various carbon blacks. The ESR spectra of natural rubber and styrenebutadiene rubber with various carbon blacks are of a complex character. A narrow component line is superimposed on the much broader signal line of carbon black. The changes in this narrow spectral line with plasticization time and with carbon black content in the rubber–filler system result from interactions between the rubber macromolecules and filler particle