Accurate knowledge of charging efficiency of particles passed through a bipolar diffusion charger is critical for accurate electrical mobility-based size distribution measurements. It is generally assumed that particles attain a steady-state charge distribution in bipolar diffusion chargers, but this assumption must be tested for its robustness. In this study, the role of flowrate on charging efficiency of particles was established as a function of particle size, charger type, charge polarity, and layout of chargers. The study only focused on singly-charged and uncharged particles entering the neutralizer. The different chargers or neutralizers studied here include Kr-85 neutralizers (TSI 3077 and TSI 3077a), a Po-210 neutralizer, and a soft X-ray charger (TSI 3087). The Po-210 and soft X-ray chargers were seen to largely charge particles to their predicted steady-state charge distribution for all flowrates and particle sizes studied. The Kr-85 neutralizers, however, show complex flow-rate dependence of charging efficiency for particles smaller than 50 nm. The flowrate dependence was non-monotonic, with a minimum charging efficiency observed at similar to 1.0 l min(-1) flow rate. For flowrates lower than 1.0 l min(-1), the decrease in charging efficiencies with increasing flowrate was shown to be because of decreasing particle residence time in the neutralizing region. At high flows (> 1.0 l min(-1)), the increase in charging efficiencies was experimentally established to be because of particle-ion interaction just downstream of the neutralizer. Thus, when Kr-85 neutralizers are used for mobility measurements, their flowrate-dependent charging efficiencies must be considered for particles smaller than 50 nm. (C) 2014 Elsevier Ltd. All rights reserved.