The nanoparticles (diameter < 50 nm) emitted by diesel engines have received increasing attention due to their potential health effects. We propose that chemiions generated during combustion play an important role in the formation of these nanoparticles. The predicted nanoparticle properties based on our chemiion theory closely match measurements in terms of total nanoparticle concentrations, and their sensitivity to fuel sulfur contents and second stage dilution conditions, while the classical homogeneous nucleation fails to explain these observed properties. Our study indicates that total number of nanoparticles formed is very sensitive to chemiion concentrations, and we propose a technique to effectively reduce vehicle nanoparticle emissions by removing ions in the exhaust. ¿ 2001 American Geophysical Union