Particle size distribution and number concentration measurements have been made in the diluted exhaust of a medium-duty, turbocharged, aftercooled, direct-injection Diesel engine using a unique variable residence time micro-dilution system that allows systematic variation of dilution and sampling conditions, and a scanning mobility particle sizer (SMPS). The measurements show that the number concentrations in the nanoparticle (Dp < 50 nm) and the ultrafine (Dp < 100 nm) ranges are very sensitive to dilution conditions and fuel sulfur content. Changes in concentration of up to two orders of magnitude have been observed when conditions are varied over the range that might be encountered in typical laboratory dilution systems. For example, at a dilution ratio of 12, dilution temperature of 32 °C, and a residence time of 1000 ms, the number concentrations reach 6 × 108 part./cm3, while at dilution ratio of 40, temperature of 48 °C, and a residence time of 1000 ms, the concentrations decrease to about 0.15 × 108 part./cm3. The use of ultra low sulfur fuel (10 PPM sulfur by mass) results in 70 % reduction of the total number emission of nanoparticles when compared with D-2 (400 PPM sulfur by mass). It is believed that the dilution conditions are influencing the nucleation rate of sulfuric acid/water particles and their subsequent growth as a result of absorption of sulfuric acid, water, and hydrocarbon species, during dilution and cooling of hot exhaust. Test Conditions that favor the formation and measurement of increased Diesel particle number concentrations are; lower dilution temperature, lower dilution ratio, longer residence time, higher relative humidity, and higher fuel sulfur content. Conversely, lower particle number concentrations are favored by; higher dilution temperature, higher dilution ratio, shorter residence time, lower relative humidity, and lower fuel sulfur content. These results show that experimental dilution conditions must be reported in detail if particle number concentrations are to be compared and interpreted.