The effect of electron beam irradiation on ethylene-propylene terpolymer (EPDM) was investigated. A 50-part oil-extended EPDM with high termonomer (ENB) content (iodine number of base polymer) 19, was selected for this study. An increase in irradiation dose from 0 to 200 kGy resulted in increased crosslinking, measured by an increase in gel contents and better swelling resistance. The effect of the multifunctional monomer trimethylol propane trimethacrylate (TMPTMA) as a crosslink promoter was studied using IR spectroscopy. The IR studies revealed enhanced peak absorbances at 1725, 1257, and 1023 cm(-1) as a result of the increased concentration of C = 0 and C-O-C groups and reduced absorbance at 1630 cm(-1) due decreased concentration of C = C groups with TMPTMA level in the irradiated samples. The presence of TMPTMA increased the level of crosslinking at a given irradiation dose, which was manifested by improvement in tensile properties. (c) 2005 Wiley Periodicals, Inc.