Diesel exhaust aerosols are highly dynamic and, therefore, difficult to sample without introducing falsification. This paper describes a study of these aerosols using a rapid dilution sampling system and an electrical aerosol analyzer. An Onan single cylinder indirect injection engine was used as an exhaust source. The sampler diluted the exhaust with clean air in ratios of 400:1 to 600:1 in order to prevent sample falsification by condensation and coagulation. The electrical aerosol analyzer was used to determine particle size and concentration. Volume concentration in the exhaust ranged from 2,000 to 50,000μ m3 cm-3 which correspond to mass loadings of 2.0 to 50 mg m-3 (assuming a density of 1 gm cm-3). Volume geometric mean diameters ranged from 0.12 to 0.19 μm.Evaporation and coagulation effects on diesel aerosols were observed by aging in a Teflon holding bag. A simple evaporation model was fit to the decrease of aerosol volume concentration with time.The fit revealed that the aerosols evaporated as if they were composed of normal paraffins in the 350-500 molecular weight range.Although the sample dilution system used in this study may alter the sample somewhat, it is probably analogous to what happens at the tailpipe of a vehicle. Measurements taken on a test track in the exhaust plume of a Peugeot 504 diesel showed aerosol size distributions very similar to those measured in our laboratory studies.