The charging and neutralization of aerosol particles by ions can take place through (1) ion attachment to the surface of aerosol particles, or else (2) electron transfer between the ions and the molecules of the aerosol particle surface. To decide which of the two mechanisms is correct, a careful experimental investigation has been carried out using ultrafine aerosol particles having electrical mobilities of the same order of magnitude as those of the ions. If the ion attachment mechanism were correct, one would expect a shift in the aerosol mobility distribution after several stages of charging and neutralization. Such a shift has not been detected in our experiments. This result supports the electron transfer mechanism as the correct one, and demonstrates that the size of ions have no effect on the size distribution of particles measured by a differential mobility analyzer, even in the extreme cases in which the sizes of ions and particles are of the same order of magnitude.