Blowout behaviors of NR/SBR blend composites reinforced with carbon black were studied using a microwave oven and variation of the blowout time and temperature with the blend ratio was investigated. Morphology of the interior of the sample before exploding was observed and change of the crosslink density was measured. The blowout time became slower and the blowout temperature became higher as the SBR content of the specimen increased. The specimen with higher NR content had more cavities in the interior just before explosion. The crosslink density became lower by coming close to the blowout and crosslink density of the inner part was more reduced than that of the outer part. New organic materials were found in the burst region after blowout and they might be decomposed products of the polymer chains. The SBR specimen showed better blowout properties than the NR one. Principal sources to Cause the blowout were found to be formation of the cavities in a rubber article, reduction of the crosslink density, and dissociation of the rubber chains. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 3627-3633, 2009