Biofuels, such as ethanol and butanol, have been the subject of significant political and scientific attention, owing to concerns about climate change, global energy security, and the decline of world oil resources that is aggravated by the continuous increase in the demand for fossil fuels. This study evaluated the potential emissions impacts of different alcohol blends on a fleet of modern gasoline vehicles. Testing was conducted on a fleet of nine vehicles with different combinations of ten fuel blends over the Federal Test Procedure and Unified Cycle. The vehicles ranged in model year from 2007-2014 and included four vehicles with port fuel injection (PFI) fueling and five vehicles with direct injection (DI) fueling. The ten fuel blends included ethanol blends at concentrations of 10%, 15%, 20%, 51%, and 83% by volume and iso-butanol blends at concentrations of 16%, 24%, 32%, and 55% by volume, and an alcohol mixture giving 10% ethanol and 8% iso-butanol in the final blend.The results showed some clear trends with increasing levels of alcohol in the blends for some pollutants, but not for others. There was a trend for lower CO, CO2, PM mass, and particle number, and lower fuel economy with higher alcohol content fuels. For other pollutants, such as THC, NMHC, CH4, and NOx, there were not strong fuel trends, while some carbonyl species showed some trends towards higher emissions for higher alcohol blends. The emissions profiles for the different vehicles also showed differences, with the wall-guided DI vehicles showing higher PM mass, and particle number compared to the PFI vehicles.