This study was carried out to determine the effect of simulated engine and catalyst malfunction conditions on the mass and composition of tailpipe particulate emissions from a 1990 Ford Taurus, running on a 3.2% oxygenated gasoline blend. Particulate emissions ranged from 2.0 to 4.4 mg/mi under all conditions studied. These emission levels are far below the current U.S. EPA standard of 80 mg/mi. Soot was a major component of postcatalyst particulate emissions (65-70%). Thermogravimetry demonstrated that only the low-to-medium boiling fraction of the engine oil was emitted from the cylinder walls and crevices. The catalyst aftertreatment significantly reduced the emissions of the low-to-medium boiling components, i.e., from 1.9 to 0.1 mg/mi. The formation of major organic compounds found in the postcatalyst particulate emissions has been explained by the thermal oxidation of aliphatic base-oil hydrocarbons. The level of ash emissions was relatively constant under all conditions (0.4-0.5 mg/mi). The particulate composition data can be useful in helping to diagnose emission control system performance.