Ethylene propylene diene monomer (EPDM) compounds were prepared with different amounts of transgenic soybean oil (TSO), a renewable and reactive plasticizer. For comparison, similar compounds were prepared with petroleum-based paraffin oil (PO), one of the most common plasticizers for EPDM. The plasticization effects of TSO and of PO were studied by Mooney viscometry, capillary rheometry, differential scanning calorimetry (DSC), and rubber processing analysis (RPA). The results showed that TSO has better plasticization effect than PO on EPDM. In addition, the curing characteristics of the EPDM compounds were studied. TSO was found to react with the curing agent dicumyl peroxide (DCP) during the curing process. Excessive amounts of TSO led to low crosslinking density, which was improved by adjusting the added amount of DCP. The mechanical properties, extraction resistance, and thermal stability of the EPDM vulcanizates plasticized with different amounts of TSO and PO were compared to determine the optimum amount of TSO to replace PO. At the same amount and no more than 15 phr of plasticizer, the TSO-plasticized EPDM vulcanizate has higher tensile and tear strength, elongation at break, extraction resistance, and thermal stability, but lower Shore A hardness than the PO-plasticized EPDM vulcanizate. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4457-4463, 2013