Diesel exhaust aerosol particle size measurements have been made by three techniques and compared. The techniques used were based on electrical mobility, diffusion, and inertial separation. The instruments used were a Thermo Systems Incorporated (TSI) Model 3030 Electrical Aerosol Analyzer, TSI Model 3040 diffusion battery in conjuction with a TSI Model 3020 Condensation Nucleus Counter, and a Univeristy of Minnesota micro-orifice impactor (0.1 μm cut size). Particle samples were also examined by electron microscopy and volatile fractions were determined by vacuum sublimation. Measurements were made on two engines, one direct injection and one indirect injection swirl chamber. Engine operating conditions were chosen to give a wide range of exhaust particle characteristics. The size measurements agreed well in the regions of instrument overlap. Measured number mean diameters varied from 0.01 μm to 0.1 μm. Total particle number concentrations measured with the electrical aerosol analyzer and the condensation nucleus counter were also in good agreement.