Alternative fuels are being evaluated in automotive applications in both commercial and government fleets in an effort to reduce emissions and United States dependence on diesel fuel. Vehicles and equipment have been operated using 100 percent biodiesel and various blends of biodiesel and diesel fuel in a variety of applications, including farming equipment and transit buses.This government study investigates the compatibility of four base fuels and six blends with elastomer and metallic components commonly found in fuel systems. The physical properties of the elastomers were measured according to American Society of Testing and Materials (ASTM) D 471, “Standard Test Method for Rubber Property-Effect of Liquids,” and ASTM D 412, “Standard Test Methods for Rubber Properties in Tension.” These evaluations were performed at 51.7°C for 0, 22, 70, and 694 hours. Tensile strength, hardness, swell, and elongation were determined for all specimens.Six metal specimens were stored for 6 months at 51.7°C. Visual inspection of the specimens was performed, and total acid number (TAN) was determined using ASTM D 664, “Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration.”The results showed reduced swelling characteristics in many materials when introduced to biodiesel and biodiesel fuel blends. Biodiesel and biodiesel fuel blends exhibited severe corrosion with copper-containing metals and high TAN'S with all metals evaluated.