Particulate matter (PM) emissions from gasoline direct injection engines are a concern due to the health effects associated with ultrafine PM. This experimental study investigated sources of PM emissions variability observed in previous tests and also examined the effect of ethanol content in gasoline on PM emissions. FTIR measurements of gas phase hydrocarbon emissions provided evidence that changes in fuel composition were responsible for the variability. Exhaust emissions of toluene and ethanol correlated positively with PM emissions, while emissions of isobutylene correlated negatively. Exhaust emissions of toluene and isobutylene were interpreted as markers of gasoline aromatic content and gasoline volatility respectively. Tests conducted with gasoline containing added toluene (10%) supported this hypothesis and led to the overall conclusion that the PM emissions variability observed can be attributed to changes in the composition of the pump gasoline being used. Tests conducted with gasoline containing added ethanol (10% and 30%) found that increasing ethanol fuel content increased PM emissions at steady-state operation, and decreased PM emissions during engine startup.