A technique is described for determining particle size distributions of diesel engine exhaust smoke using an electrical aerosol analyzer. Diesel smoke is a highly concentrated aerosol. As such, it is a dynamic system undergoing rapid changes as a result of condensation and coagulation processes. Consequently, great care must be taken to prevent falsification when sampling this system. A dilution probe has been developed which rapidly dilutes an exhaust sample with clean air, effectively freezing condensation and coagulation processes. The diluted exhaust is collected in a teflon holding bag and subsequently analyzed. An electrical aerosol analyzer is used to determine the particle size distribution in the 0. 0075 to 0. 6 mu m diameter size range. The data allow total aerosol volume concentrations and (assuming unity particle density) mass concentration to be determined as well.