Alternative fuels for internal combustion engines have been the subject of numerous studies. The new U.S. Renewable Fuel Standard has made it a requirement to increase the production of ethanol and advanced biofuels to 36 billion gallons by 2022. Because corn-based ethanol will be capped at 15 billion gallons, 21 billion gallons must come from the advanced biofuels category. A potential source to fill the gap may be butanol and its isomers as they possess fuel properties superior to ethanol. Recently, concerns have been raised about emission of currently non-regulated constituents, aldehydes in particular, from alcohol-based fuels.In an effort to assess the relative impact of the U.S. Renewable Fuel Standards on emissions from a modern gasoline engine, both regulated and non-regulated gas constituents were measured from the combustion of three different alcohol isomers in a modern direct-injected (DI) spark ignition (SI) gasoline engine. Exhaust gas recirculation (EGR) was disabled to avoid changes in emissions due to slight changes in EGR ratio, thereby allowing direct comparison of the emissions results. A standard emissions bench in combination with a Fourier Transform Infrared (FTIR) analyzer was used to characterize the exhaust stream before catalyst in gasoline operation as well as operation using several gasoline/alcohol blends. Ethanol, n-butanol and iso-butanol were used as blending agents. Relevant exhaust constituents were measured as a function of blend ratio for several engine load and speed conditions.Oxides of nitrogen emissions decreased with increased alcohol content while formaldehyde and acetaldehyde show a clear, positive correlation with blend ratio. Although an apparent reduction in aromatic hydrocarbon emissions was observed with increased alcohol content, this was likely caused in large part by dilution of aromatics (high volume percent in gasoline) in the fuel blend rather than changes in the fuel chemistry due to combustion. Examination of the major precursors (propene, 1,3-butadiene, and acetylene) to benzene, and therefore particulate matter (PM), revealed significant increases consistent with iso-butanol emissions poised to produce benzene via the C₃/C₃ route, whereas n-butanol emissions appeared primed to proceed through the C₄/C₂ route.