Physical characterization of the exhaust aerosols produced by two diesel-powered automobiles was done using the Mobile Air Pollution Laboratory (UMML) built by the University of Minnesota Particle Technology Laboratory. The two automobiles were a VW Rabbit and an Oldsmobile 5.7 liter V-8. Plume dilution ratios were measured using NOx as a tracer gas.Samples were taken in the exhaust plume at a point approximately one second after exhaust. The dilution ratios ranged from about 630 at idle to about 5000 at 80 km per hour. Experiments were performed at steady state road load only.Aerosol number, surface and volume concentration, and size distribution were measured using an electrical aerosol analyzer contained in the UMML. The total number concentration of submicron aerosol was measured using a condensation nuclei counter. Tailpipe number concentrations ranged from 2.4 x 108 to 2.2 x 109 particles · cm-3. Volume concentrations ranged from 3 x 104 to 2.1 x 105 μm3 · cm-3. The size distributions indicated a volume mean diameter of about 0.2 μm.General characteristics of the roadway aerosol size distribution were similar to those measured when using a jet pump diluter in laboratory dynamometer studies.