Polymerized soybean oils of different molecular weights were used as plasticizers in blends of natural rubber with styrene butadiene rubber. The oils of different molecular weights and viscosities were synthesized by cationic polymerization using a proprietary technology. Since vegetable oils have double bonds, they are not only viscosity depressants but they may be active participants in crosslinking 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 polymerized soybean oil could be substituted for naphthenic process oil with minimal differences in mechanical and dynamic properties.