Abstract : Diesel fuels derived from petroleum and synthetic sources were compared chemically and toxicologically to address the issue of whether a changeover of mobility fuels from natural to synthetic sources might be accompanied by greater or different toxicological hazards to military personnel. Diesel fuels derived from petroleum, shale oil, tar sands, and tar sands/ petroleum coprocessing were compared for major organic composition, 4-6 ring polycyclic aromatic hydrocarbon dermal tumorigens, major organic compounds in their inhalable volatiles, mouse dermal tumorigenicity and promoting activity. The database was expanded by a similar comparison of petroleum- and coal-derived fuels for the U.S. Department of Energy (USDOE). The results suggest that highly refined and finished mobility fuels from synthetic or alternate sources will not pose a significantly greater toxicological hazard than current petroleum-based fuels. Toxicological differences are likely to be subtle. Keywords: Mobility fuels, Natural and synthetic, DF-2, Chemistry, Toxicology, Petroleum, Shale oil, Diesel fuel.