Polymerized soybean oils of different molecular weights were used as plasticizers in NR/SBR compositions. The oils of different molecular weights and viscosities were synthesized by cationic polymerization using a proprietary technology. Because vegetable oils have double bonds, they are not only viscosity depressants but also active participants in cross-linking reactions. Properties of elastomers extended with different concentrations of mineral oil or pure soybean oil were compared with elastomers extended by polymerized oils of different molecular weights at the same concentrations. It was found that polymerized soybean oil could be substituted for naphthenic process oil with minimal differences in mechanical and dynamic properties.