Experiments were performed to measure the number-weighted particle size distributions emitted from a gasoline direct injection (GDI) engine. Measurements were made on a late model vehicle equipped with a direct injection spark ignition engine. The vehicle was placed on a chassis dynamometer, which was used to load the engine to road load at five different vehicle speeds ranging from 15 - 100 km/hr. Dilution of the exhaust aerosol was carried out using a two-stage dilution system in which the first stage dilution occurs as a free jet. Particle size distributions were measured using a TSI 3934 scanning mobility particle sizer.Generally speaking, the presence of the additives did not have a strong, consistent influence on the particle emissions from this engine. The polyether amine demonstrated a reduction in particle number concentration as compared to unadditized base fuel. A polyolefin amine (as well as one of the proprietary additives) did produce greatly increased emissions of particles larger than 100 nm at the 15 and 30 km/hr operating conditions. The second proprietary additive was very effective at reducing particle emissions; it reduced particle number emissions by an average of 31%, and by 44% at a highway cruising condition of 100 km/hr.The free-jet dilution used in this set of experiments resulted in number concentrations which were similar to those measured for this engine using a dual-stage forced mixing dilution system in a previous study. The size distributions obtained using the current free-jet dilution system were biased toward much smaller particles (geometric mean diameter in the range from 25 - 35 nm) than was the case for the forced mixing dilution system (which had size distributions with typical geometric mean diameters of 75 - 85 nm). This reduction in particle size is likely due to the decrease in time available for particle growth processes such as agglomeration and condensation when using the free-jet dilution system.