The present study is a detailed analysis of the charging performance of aerosol nanoparticles using soft Xray photoionization. The study includes methods used to suppress the formation of unnecessary nanoparticles during ionization. Using a soft X-ray ionizer, the charging performance was evaluated at different emitting energies (3, 4 and 9.5 keV) and for ionization time and chamber type. The results were compared with the charging performance of an Am-241 alpha-ray ionizer. The experimental results showed that the total charging ratio for monodispersed nanoparticles is similar between X-ray and alpha-ray ionizers. However, a considerable amount of nanometer-sized particle formation caused by X-ray photoionization also was detected. Particle generation was significantly dependent on gas residence time in the ionization chamber, which was caused by ion-induced nucleation. Overall, the soft X-ray photoionizer is a more suitable bipolar diffusion charger because of a superior capacity for charging large numbers of dense aerosol particles in a relatively short residence time, which can be controlled in the ionization chamber. (C) 2009 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.