The rapid expansion of a supercritical solution (RESS-process) is modelled numerically to better understand the mechanisms of particle formation and growth. A model is presented which includes the expansion process inside the capillary nozzle as well as the post-expansion process with particle growth inside the expansion chamber. Results are shown for the solid benzoic acid, griseofulvin and beta-sitosterol in combination with the solvent carbon dioxide and trifluoromethane. The comparison with experimental results shows good agreement in general trends but does not yet match exactly with the measured mean particle sizes. Overall, a powerful tool for the optimisation of the RESS-process has been created which contributes to the understanding of the RESS-process. (C) 2002 Elsevier Science B.V. All rights reserved.