Polymerization of soybean oil produces higher-viscosity liquids, which may serve as processing aids and plasticizers in certain rubbers as a replacement of petrochemical oils. Four polymerized soybean oils of different molecular weights showed good compatibility with ethylene–propylene–diene rubber (EPDM), but because of the presence of double bonds and copolymerization with EPDM, they decreased the cross-linking density when compared with paraffinic extender oil. As a consequence, polymeric soybean oils reduced tensile strength and modulus but increased elongation, tear strength, and compression set. Higher-molecular-weight plasticizers are expected to reduce sweating out of oils. Pure soybean oil was not completely compatible at the concentration tested, but it showed a strong plasticizing effect; dramatically lowered tensile strength, tear strength, and modulus; and increased elongation and compression set. No clear effect of molecular weight of polymerized soybean oils on properties was observed, but increasing the sulfur content was found to be beneficial. Using polymeric vegetable oils instead of petrochemical extenders in EPDM rubbers is economical and environmentally desirable, but the curing system requires optimization to accommodate loss of cross-linking density.