The bipolar diffusion charging of aerosol particles in the transition and continuum regimes was studied theoretically and experimentally under the conditions of high particle/ion concentration ratios. The basic equations for the bipolar diffusion charging of monodisperse and polydisperse aerosols were numerically solved for geometric mean diameters of 0.01–1.0 μm, geometric standard deviations of 1–2, and for various concentrations of bipolar ions. From the calculation results presented in terms of dimensionless parameters, the ion number concentrations and charging times required to attain the equilibrium charge distributions were determined. In the experiment, the changes in the particle charge distribution and ion number concentration in the flow-type chamber were measured for various particle number concentrations, ion generation rates, and residence times. For bipolar ion generation rates > 3.5 × 1011 m−3 s−1, the experimental results were found to agree with the theoretical predictions. For smaller ion generation rates, however, the particles were not charged as predicted by theory.