A single-cylinder, 0.36ℓ, D.I. Diesel engine was operated on Diesel fuel, sunflowerseed oil, cottonseed oil, soybean oil, and peanut oil. The purpose of this study was to provide a detailed comparison of performance and emissions data and to characterize the biological activity of the particulate soluble organic fraction for each fuel using the Ames Salmonella typhimurium test. In addition, exhaust gas aldehyde samples were collected using the DNPH method. These samples were analyzed gravimetrically and separated into components from formaldehyde to heptaldehyde with a gas chromatograph.Results comparing the vegetable oils to Diesel fuel generally show slight improvements in thermal efficiency and indicated specific energy consumption; equal or higher gas-phase emissions; lower indicated specific revertant emissions; and significantly higher aldehyde emissions, including an increased percentage of formaldehyde.