The reaction mechanism of ethylene-propylene-diene (EPDM) peroxide vulcanization, in the presence of triallylcyanurate (TAC), as a coagent, was studied by IR spectroscopy and equilibrium swelling technique. Coagent TAC is added in EPDM polymer to prevent the scission of polymer chains during the curing process. Because of this incompatibility with polymer, TAC was supposed to reside in small domains. To establish the existence of coagent bridges, the cleavage of ether linkages in the vulcanizates was made. After that treatment, IR measurements proved incorporation of coagent in polymer network. Measurements of allyl absorbances and the s-triazine ring indicated that crosslink efficiency is enhanced and homopolymerization and cyclization take place in the presence of the coagent. In addition, the swelling results and IR measurements confirmed that the properties of the polymer, such as molecular mass and molecular mass distribution, will determine which of these reactions will predominate during the vulcanization process.