ABSTARCTSynthetic middle distillate fuels produced by a Fischer-Tropsch process are one of the many long-term alternatives to petroleum-derived fuels that are under active consideration. Agricultural-based oils have been considered a short-term solution and are currently being used in petroleum fuel blends. This article shows that agricultural-based biodiesel blended with synthetic fuels, such as those derived by a Fischer-Tropsch process, are chemically unstable. This same biodiesel blended with petroleum diesel demonstrates a high degree of chemical stability. The gc/ms profile of a Fischer-Tropsch middle distillate fuel compared with petroleum-derived middle distillate fuels shows absence of aromatic content in the Fischer-Tropsch middle distillate fuel. This absence in aromatic content explains the instability and solubility problems observed between Fischer-Tropsch/biodiesel blends.