Peroxide cure of ethyl propiolate-grafted isobutylene-isoprene rubber (EIIR) was studied with the goal of replacing the toxic m-phenylene-N,N'-bismaleimide (BMI) with the less toxic trimethylolpropane trimethacrylate (TMPTMA) as the curing coagent. Dicumyl peroxide (DCP) failed to sufficiently cure EIIR in the presence of TMPTMA, whereas benzyol peroxide (BPO) was effective. The EIIR vulcanizates cured by the BPO-TMPTMA combination displayed further improved mechanical properties in comparison to those of the EIIR cured by DCP and BMI. The balanced stiffness, strength, and extensibility of the BPO-TMPTMA-cured EIIR are the best among all peroxide cured IIR derivatives and compare favorably against those of sulfur-cured IIR derivatives that are not reinforced by fillers. The outstanding mechanical properties are attributed to reinforcement of the elastic continuous phase by TMPTMA-rich hard domains. The existence of such hard domains was confirmed by transmission electron microscopy and atomic force microscopy. The peroxide-cured EIIR composite reported here is potentially useful for pharmaceutical closure applications for its simple nontoxic formulation and excellent mechanical properties.