Silica-filled styrene-butadiene rubber (SBR) unvulcanizates and their vulcanizates with different rubber/filler interactions were prepared by using several kinds of coupling agents. Tensile tests and electron spin resonance (ESR) measurements were carried out for both unvulcanized and vulcanized samples to get information on the effects of fitler/rubber interactions on the breakdown of carbon-carbon (C-C) linkages in SBR and carbon-sulfur (C-S-C, C-S-S-C) linkages at the crosslinked points between rubber and sulfur by a tensile force. The combination of ESR results and stress-strain data suggested that with increasing the mechanical energy applied to the samples by the stretching, the carbon-sulfur linkages around silica particles were broken first, followed by the breakdown of carbon-sulfur and C-C linkages in the rubber matrix. The assignment of ESR spectrum was also discussed. (c) 2008 Wiley Periodicals, Inc.