The petroleum fraction known as middle distillate that is used for transportation purposes consists of diesel, jet fuels. Many agricultural-based oils have been considered and used in petroleum fuel blends or as petroleum replacements. Soybean-derived biofuels have proven to be an excellent substitute for petroleum diesel fuel applications, such as cars, trucks, trains, and ships. However, biofuel replacements have not been studied for long-term storage tank stability. This type of storage stability would involve mixtures with not only biofuel but also water and air. The tank contents would be in constant contact with water bottoms and air because of draw down and refilling of partially empty storage tanks. The results of this study are important because the Department of Defense will be purchasing diesel fuel that contains up to 5% biodiesel. This article examines the storage tank stability, by ASTM method D-5304, of soy-derived biodiesel petroleum blended fuel in an environment of air and dissolved water.