Blowout behaviors of carbon black-reinforced SBR composites were studied using amicrowave oven and influence of the oil content and the SBR type on the blowout behaviors was investigated. Morphology of the interior of the sample before exploding was observed and change of the crosslink density was also measured. The blowout time was faster and the blowout temperature was lower for the specimen with high oil content than for the specimen with low oil content. The blowout time and temperature of the specimen made of high molecular weight SBR were slower and higher, respectively, than those made of low one. The specimen with high oil content had lots of cavities in the interior before exploding. It was found that the interior morphologies before explosion were consistent with the burst morphologies as well as the blowout times and temperatures. The crosslink density became lower by closing to the blowout and crosslink density of the inner part was more reduced than that of the outer part. New organic materials were generated in the burst region by decomposition of the rubber chains. Major sources to cause the blowout were found to be activation of the cavity formation, reduction of the crosslink density, and dissociation of the rubber chains.