Several synthetic fuels derived from shale and coal were evaluated with respect to a reference petroleum-based Diesel fuel. Tests conducted using a single-cylinder DI Diesel engine were designed to quantitatively compare the fuels on the basis of performance, combustion characteristics, gas-phase emissions, particulate emissions, and biological activity of the solid phase soluble organic fraction. The biological activity was assessed using the Ames Salmonella typhimurium test. The shale fuels studied were a Paraho marine Diesel fuel and a light shale oil condensate from the Logan Wash in situ retorting operation. The coal liquids, Solvent Refined Coal-II and Exxon Donor Solvent, could not be run neat; therefore, they were blended 20% and 40% by volume with the certified DF-2 baseline fuel. Of the synthetic fuels tested, only the Paraho marine Diesel fuel exhibited the qualities of a good finished Diesel fuel.