The density of nanometer sized particles of different materials (Pd, Ag, Co, Carbon, oil droplets and oil burner exhaust) suspended in a carrier gas is determinedinsituas function of particle radius via the measurement of particle mobility and inertia using a differential mobility analyzer and a low pressure impactor. From the density and size of agglomerated particles, information on the structure of the agglomerates can be obtained; in particular a fractal‐like dimension can be determined. Whereas the densities of carbon particles and of particles in oil burner exhaust have the expected values, the densities of the metal particles are too low compared to the bulk values. The results for the fractal‐like dimensions of the agglomerates can be explained by assuming a mixture between monomer‐cluster and cluster‐cluster agglomeration in the free‐molecular regime. The fractal‐like dimension of ferromagnetic Cobalt agglomerates provides evidence that particles consisting of magnetic materials tend to form chainlike agglomerates. ©1995 American Institute of Physics.