We investigate the effect of the vapour polarity on the mobility and size of ion clusters. The interaction of the neutral polar molecules with the electrical field of the charged particle/cluster may lead to significant change in the theoretical particle diameter and the corresponding mean ion mobility. The observed deviation between the theoretical Kelvin-Thomson diameter and the Millikan-Fuchs mobility equivalent diameter derived from the experimental data is explained. We suggest that in the size range of similar to1-2 nm the classical Millikan-Fuchs relationship remains valid and propose a modified version of the Kelvin-Thomson equation accounting for the dipole-charge interaction. (C) 2003 Elsevier B.V. All rights reserved.