The charged fraction of ultrafine silver particles (5–50 nm) suspended in high-purity helium (99.998% grade) was measured as a function of particle size under different charging conditions (i.e., ion concentration and charging time). The charger used is a modified version of the one described in a previous study (Romay et al., 1991). Two radioactive sources of 250 μCi Po-210 are located in a cavity at the charger inlet. The cavity configuration is designed to control the range of the α rays. The modified charger provides a uniform electric field in the axial direction to separate the positive ions from the negative ions and electrons produced by the ionizing radiation. Thus, the aerosol particles are first exposed to a small region of electrons and positive ions, and are subsequently charged by positive ions in a much longer section of the charger. The ion concentration is estimated from the ion current collected in the exit electrode and measured with a picoammeter. The charging time is estimated from the length of the unipolar section of the charger and from the flow velocity profile in the charger. Experiments were performed to measure the fraction of uncharged particles by using a condensation particle counter equipped with a multichannel analyzer. From the measured uncharged fraction, ion concentration and charging time, the combination coefficient between positive ions and neutral particles was determined. The Knudsen number of the experimental data ranged from 2.4 to 24. The results were compared with the combination coefficient calculated using several available theories. It is concluded that Fuchs’ limiting-sphere theory (1963), using ion properties of He2+gives the best agreement with the experimental results. This is Particle Technology Laboratory Publicati No. 807.