Technical Abstract: Biodiesel, an alternative fuel made from transesterification of vegetable oil with methanol, is becoming more readily available for use in blends with conventional diesel fuel for transportation applications. Biodiesel has fuel properties comparable to those of conventional diesel fuel and is known to enhance ignition quality (cetane number) and lubricity characteristics in blends. Biodiesel reduces harmful exhaust emissions including particulate matter, volatile organic compounds, polycyclic aromatic hydrocarbons, carbon monoxide and smoke. Consequently, blending biodiesel with jet fuel for aviation applications may have potential for providing comparable performance while reducing ground level emissions. Given that production of biodiesel in the U. S. grew from 500,000 gal in 1999 to 25 million gal in 2001, storage stability with respect to oxidative degradation will increase in significance as a concern for fuel suppliers and terminal operators. This work investigates effects of soy biodiesel (SME) on the oxidative stability of jet fuel (JP-8). Oil stability index (OSI) data (in h) were measured under isothermal conditions for 10, 20, and 30 vol. percent SME-blends in accordance with the Am. Oil Chem. Soc. standard method Cd 12b-92. For 10 percent blends, OSI = 331.1, 192.0, 100, and 46.3 h with respect to block temperatures = 50, 60, 75, and 90 deg. Celsius. Increasing blend ratio of SME decreased OSI. Finally, a block temperature of 90 deg. Celsius is recommended when measuring OSI as an expeditious means for assessing the relative resistance to oxidation of SME/JP-8 blends.