Gasoline direct injection (GDI) engines have improved thermodynamic efficiency (and thus lower fuel consumption) and power output compared with port fuel injection (PFI) and their penetration is expected to rapidly grow in the near future in the U.S. market. In addition, the use of alternative fuels is expanding, with a potential increase in ethanol content beyond the current 10%. Increased emphasis has been placed on butanol due to its more favorable fuel properties, as well as new developments in production processes.This study explores the influence of mid-level ethanol and iso-butanol blends on criteria emissions, gaseous air toxics, and particulate emissions from two wall-guided gasoline direct injection passenger cars fitted with three-way catalysts. Emission measurements were conducted over the Federal Test Procedure (FTP) driving cycle on a chassis dynamometer. This study utilized seven fuels with varying ethanol and iso-butanol contents, including E10, E15, E20, Bu16, Bu24, Bu32, and a mixture of E20 and Bu16 resulting in E10/Bu8. Emissions included nitrogen oxides (NOx), carbon monoxide (CO), total hydrocarbons (THC), non-methane hydrocarbons (NMHC), methane (CH4), and carbon dioxide (CO2). Additionally, carbonyl compounds, 1,3-butadiene, benzene, ethylbenzene, toluene, and xylenes were quantified in the exhaust. Total particle number emissions, black carbon, and real-time particle size distributions were also measured. The results are discussed in the context of the changing fuel type and composition.