Particles in the size range of a few nano-meters are characterized by means of a differential mobility analyzer (DMA) of the Eichler type in tandem with a focusing impactor with electrostatic blowing. One application of the DMA-impactor combination is determination of particle mass m(p) without knowledge of the relation Z(d(p)) between particle mobility Z and diameter d(p). With Z(d(p)) known, also. the size and density rho(p) of spherical particles can be determined. A mobility versus diameter relation Z(d(p)) for the range of a few nanometers is derived fro in the literature. It considers the effect of the finite diameter d of gas molecules, neglected in conventional studies. According to this relation, plots of Z versus mp yield straight lines, and the intersection with, the Z(-1/2) axis is linearly related to the effective diameter d of the carrier gas molecules. Experimental data recorded with the DMA-impactor combination for silver particles are consistent with this relation. The same applies for values from literature, for fullerenes and proteins. For air, d is approximately 0.53 nm, not far from the 0.6 nm estimated by Tammet (1995) The particle density rho(p) is derived from mp and the diameter inferred from Z(d(p)). Measurement of rho(p) on silvers particles showed that charging devices may introduce contamination leading to formation of an adsorbed layer on the particles, reducing their measured density. The DMA-Impactor combination applied has sufficient resolving power to observe occurrence of different shapes of particles prepared by vapor condensation. (C) 2002 Elsevier Science Ltd. All rights reserved.