A one-dimensional flow model has been proposed to describe the particle formation through the rapid expansion of supercritical solutions (RESS). This model includes the wall friction in the nozzle and heat exchange with the surrounding in the supersonic free jet region. The calculations were performed on the ibuprofen-supercritical CO2 system in terms of the supersaturation, nucleation rate, critical nucleus size and number concentration of the critical nuclei. The results show that very high supersaturation (approximately 10(9)), thus high nucleation rate, can be attained when the fluid leaves the nozzle exit to the supersonic free jet region. Simultaneously, the critical nucleus size, in the nanometer size, decreases, whereas the number concentration of critical nuclei increases. Besides, sensitivity analyses of pre-expansion pressures, pre-expansion temperatures and nozzle length have been carried out but found that they seem to not significantly influence the flow field properties. (c) 2005 Elsevier B.V. All rights reserved.