Results are presented, concerning the formation, the size distribution, and the behavior of ultrafine particles produced by a disintegration of actinon and UV irradiation in filtered and natural atmospheric air. The characterization of these particles is obtained by electrical aerosol analyzer and diffusion battery method. Measurements are made in the range betwen 0.003 and 0.5 μm. Some qualitative indications are obtained on the different mechanisms which govern the evolution of ultrafine particles in the atmosphere (nucleation, coagulation, and condensation).