To investigate the sources to cause blowout of a rubber article gaseous materials (blowout gas) generated from the NR and SBR composites by blowout and organic materials extracted from the burst region after explosion of the specimen were analyzed using GC/MS. 1-Methyl-4-(1-methylethenyl)-cyclohexene was detected from the blowout gas of the NR vulcanizate and 4-vinylcyclohexene and styrene were detected from the blowout gas of the SBR vulcanizate. They, were the repeat units of the used rubbers and were produced by decomposition of the polymer chains. Organic additives contained in the initial specimen were not detected in the burst region after blowout but lots of hydrocarbon components were detected instead. We believe that a principal source to cause the blowout was decomposition of the polymer chain, and it leads to activation of the cavity formation in the rubber article and reduction of the resistance to the increasing interior pressure. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110:3068-3072, 2008