In the course of a vehicle dynamometer study of sulfur emissions from an automobile equipped with oxidation catalysts, aerosol size distribution measurements were made while the exhaust was rapidly mixed with 10 parts of filtered air in a dilution tube. Results obtained with an electrical aerosol analyzer show that, under these dilution conditions, the sulfuric acid aerosols are consistently smaller than 0.3 μm in diameter. The distribution of aerosol volume (or mass) is slightly bimodal with maxima near 0.03 and 0.09 μm particle diameters. When a light-scattering effect is computed from theory using the observed particle size distributions good agreement is obtained between theory and measured light scattering. Within a reasonable margin of error, consistency is found between cascade impactor data, nephelometer light-scattering data, electrical analyzer data, and chemical analyses for sulfate.