The charge neutralization process for aerosol particles in a bipolar ionic atmosphere has been studied theoretically and experimentally. The degree of neutralization was found to depend on the parameter, Nt, where N is the ion concentration and t is the neutralization time. For particles in the 0·001-100, μm dia. range, the maximum Nt product needed for charge neutralization was found to be 6 × 106 and 1 × 105 (ions/ml) (sec) respectively for the two limiting cases studied, i.e. a ≫ λ and a < λ, where a is the particle radius and λ is the mean free path of the small ions. Experimental studies were made on four radioactive krypton-85 neutralizers with activities of 0·5,1, 2 and 10 mCi. The equilibrium ion concentration was found to range from 1·2 × 106 to 8·8 × 106 ions/ml. The performance of these devices as aerosol charge neutralizers was investigated using monodisperse DOP (di-octyl phthalate) aerosols of 2·53 μm dia. The results are in good agreement with the theoretical predictions.