Biodiesel is a fuel consisting of the alkyl monoesters of vegetable oils or animal fats. Biodiesel is nontoxic, renewable, and biodegradable. Biodiesel-fueled engines produce less carbon monoxide, unburned hydrocarbon, and particulate emissions than diesel fueled engines. One drawback of biodiesel is that it is susceptible to oxidation which can induce polymerization of the esters and can form insoluble gums and sediments which are known to cause fuel filter plugging. However, no research has been conducted to determine the impact of oxidized biodiesel on engine emissions and fuel system performance;The objective of this study was to relate the chemical and physical processes associated with biodiesel oxidation to the conditions that affect engine performance and emissions. In addition, a relationship was sought between ASTM D2274, a diesel fuel-based stability test and AOCS Cd 8-53 and Cd 3a-63 which characterize the chemical changes in the fuel;It was expected that the fuel filters would plug as the vegetable oil esters oxidized but no filter plugging was observed in this study even when the fuel oxidized beyond the level that would be encountered in practice. Recent research by others has suggested that the filter plugging may be associated with reactions between the diesel fuel additives and biodiesel;The engine performance of the oxidized biodiesel was similar to that of No. 2 diesel fuel with nearly the same thermal efficiency, and slightly higher fuel consumption. Oxidized biodiesels produced between 14% and 16% lower CO and HC emissions and smoke number compared to unoxidized biodiesel. No statistically significant difference was found between the NOx emissions from oxidized biodiesel and unoxidized biodiesel. Oxidized biodiesel experienced a one degree shorter ignition delay than unoxidized biodiesel. The ignition delay was almost linearly correlated to CO and HC emissions. A common linear relationship was found between the start of combustion and the NOx emissions. When the NOx was plotted against the start of combustion timing, the neat biodiesel produced lower NO x emissions than the No. 2 diesel fuel.