An experimental study of charging and neutralization of 2.5-10 nln aerosol! particles by bipolar air ions has been carried out. The time required to attain the charge equilibrium slate, both for charging of initially uncharged particles and for neutralization of charged particles, has been previously determined using different-sized diffusive type chargers. To avoid sizing errors caused by space charge field and Brownian diffusion effects, a tandem DMA (differential mobility analyzer) system has been used. The first DMA classifies a certain fraction of particles, and the second one allows precise determination of their size. The experimental charging probabilities at equilibrium are in relatively good agreement with calculations based on Fuchs＇ theory, except in the particle size range below 3 nm where large deviations have been observed.