Droplet growth by heterogeneous condensation of water vapour as a preconditioning technique for the separation of submicron particles from gases has been investigated both theoretically and experimentally. The fundamentals of droplet growth theory including the growth of soluble particles are presented. From the results of numerical calculations, the influence of supersatura tion, particle concentration, temperature and initial size distribution on the activation and growth rate of the particles is demonstrated. Furthermore, experimental results are reported and compared with the numerical results. Both theoretical and experimental results show that with a mass of condensable water vapour of ca. 5.5 g m(-3), submicron particles can be enlarged with high growth rates to droplets with mean diameters of ca. 3 mu m. Hence heterogeneous condensation in combination with a standard inertial separator, e.g. a cyclone, is a highly efficient technique for separating submicron particles. Additionally, some results for the growth of soluble particles, in this case NaCl particles, are presented.