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Peroxide curing of brominated butyl rubber (BIIR) is an attractive topic, but the degradation of BIIR during the curing is a drawback needed to be overcome. Coagent assisted peroxide curing system is an attractive and effective choice in order to increase the crosslink density of rubbers. 1,2-polybutadiene (1,2-PB) is used as a crosslinking coagent for the curing of BIIR by dicumyl peroxide (DCP), and the effect of 1,2-PB on the curing characteristics, crosslink density, and mechanical properties is investigated. The addition of 1,2-PB affects the curing characteristics of BIIR compound and significantly increases the crosslink density of BIIR vulcanizates. With increasing 1,2-PB content, the tensile strength and stresses at a given extension of BIIR vulcanizates increase, but the elongation at break decreases. A stress-softening effect of the carbon black filled BIIR vulcanizates is observed and becomes more pronounced with increasing 1,2-PB content. The addition of 1,2-PB increases the stress relaxation index of BIIR. GPC and 13 C-NMR results indicate 1,2-PB participates in the crosslinking reaction, and the existence of 1,2-PB component in the insoluble fraction of BIIR/1,2-PB vulcanizates is confirmed by solid-state 13 C-NMR.