Two synthetic Diesel fuels, one derived from oil shale and the other from tar sands, were compared to a petroleum-based number-two Diesel fuel. These fuels were tested in a single-cylinder, air-cooled, direct-injected, light-duty Diesel engine. Comparisons were made on the bases of performance, combustion characteristics, gas-phase emissions (including aldehydes), and particulate emissions. The aldehyde emissions were measured using the DNPH method with a gas-chromatographic finish, while the mutagenic activity of the particulate emissions soluble organic fraction was assayed using the Ames Salmonella typhimurium test.The shale-derived fuel, manufactured by Suntech, Inc., was moderately hydrotreated, producing a cetane number of 51. The shale fuel exhibited behavior similar to that of the petroleum-derived number-two baseline fuel. The National Research Council 1990 Diesel fuel was derived from a mixture of conventional Alberta and Syncrude tar sands crude stocks, and then blended with a hydrotreated catalytically-cracked cycle oil. This fuel exhibited behavior commensurate with its reported cetane number of 35.