The introduction of biofuels is seen as a very important measure to reduce the emissions of greenhouse gases from the transport sector. Currently, ethanol is the most widely used renewable fuel for transportation in the US and with the push to use increasingly higher levels of renewable fuels, there has been an accompanying push to further increase the ethanol level in gasoline. In addition to ethanol, butanol, an alcohol which can be produced from biomass sources, has recently received more attention as an alternative to gasoline for use in spark ignition (SI) engines.For this study, two 2007 model year and one 2012 model year light-duty vehicles equipped with a three-way catalyst (TWC) were employed. For the 2007 model year vehicles, emissions and fuel economy measurements were made for E10 (reference fuel), E15, E20, and B16 fuels. The latter corresponds to a blend of gasoline and 16% of butanol, which is the equivalent of E10 in terms of oxygen content. For the 2012 passenger car, in addition to E10, E15, E20, and B16, emissions and fuel economy tests were also made for E10/B8. The alcohol mixture of E10/B8 is equivalent of E15 in terms of oxygen content. Emissions measurements were performed over the Federal Test Procedure (FTP) and the California Unified Cycle (UC) test cycles for each vehicle/fuel combination on a chassis dynamometer. Emissions included nitrogen oxides (NOx), carbon monoxide (CO), total hydrocarbons (THC), non-methane hydrocarbons (NMHC), methane (CH₄), and carbon dioxide (CO₂). Additionally, carbonyl compounds were also quantified in the exhaust for all vehicle/fuel combinations over the FTP cycle. Particle size and number were measured with a scanning mobility particle sizer (SMPS) in tandem with a condensation particle counter (CPC). The concentration of black carbon was also measured with a Multi-Angle Absorption Photometer (MAAP).