Abstract We evaluated the secondary aerosol formation from two current technology flex fuel vehicles (FFVs) equipped with gasoline direct injection engines when operated on different fuel formulation on a chassis dynamometer. This study explored the influence of ethanol fueling and aromatic concentration on secondary organic aerosol (SOA) production by utilizing two E10 fuels with different aromatic levels and two higher ethanol blends (i.e., E30 and E78). The diluted emissions from both FFVs were photooxidized in a 30 m3 mobile atmospheric chamber with two distinct initial conditions, while the vehicles operated over cold-start and hot-start LA92 test cycles. Testing revealed that one vehicle did not produce secondary aerosol when emissions were photooxidized with a single precursor oxidant. However significant SOA formation occurred in the presence of an anthropogenic volatile organic carbon surrogate and NOx mixture in the chamber, despite the fact that both vehicles had similar primary particulate emission levels. The secondary aerosol formed in an urban airshed was mainly composed of ammonium nitrate and significantly exceeded primary tailpipe PM emissions. Cold-start operation from both vehicles led to greater secondary aerosol production compared to hot-start LA92 operation. SOA production showed a strong relationship with the tailpipe non-methane hydrocarbon (NMHC) emissions and overall reduced with higher ethanol blending in the fuel. The impact of aromatics was clear, with the higher aromatic E10 fuel showing higher SOA formation than the lower aromatic E10 fuel.