A theoretical investigation has been carried out to examine the effect of the initial ion-aerosol mixing state on the resulting charged fraction (extrinsic charging efficiency) of ultrafine particles in a circular tube under laminar flow conditions. Particle and ion loss to the walls by diffusion and electrostatic dispersion were taken into account in the calculations. Four different mixing states were considered at the inlet section of the charging tube: (I) uniform mixing of ions and neutral particles; (II) a core of ions surrounded by an envelope of neutral particles; (III) a core of neutral particles surrounded by an envelope of ions; and (IV) the inlet section split up into two circular segments, one containing neutral particles alone, the other only ions. The numerical results show that the extrinsic charging efficiency strongly depends on the initial mixing state, the maximum efficiency being attained for mixing state II, a state which can be easily realized in practice by injecting ions and particles into the charger through two coaxial cylinders. The worst case is that of mixing state 111, the reverse of 11, in which the neutral particles are injected through the inner cylinder, and the unipolar ions through the annular space between the injection tubes. The main reason underlying the observed behavior is the strong dependency or the ions loss rate to the wall on their initial spatial distribution. (C) 2009 Elsevier Ltd. All rights reserved.