This paper presents results of an experimental investigation on gas-phase nucleation with and without seed particles. First, experiments of titanium oxide particle formation by gas-phase hydrolysis of titanium tetraisopropoxide have been carried out to find out the average size and number concentration of homogeneously nucleated particles as a function of the operating conditions. Second, the effect of the presence of seed particles on homogeneous nucleation suppression has been examined with experiments of DOS nucleation using ZnCl2 particles as seeds. A simple model of homogeneous and heterogeneous nucleation is also presented which enables estimation of the relative importance of homogeneous and heterogeneous nucleation as a function of the size and number concentration of seed particles. In the limiting case of no seeds, the model leads to some qualitative relations between monomer concentration and size and number concentration of the homogeneously nucleated particles, which are in good agreement with the experimental results. (C) 2000 Elsevier Science Ltd. All rights reserved.