The development of nucleation theory makes possible to predict droplet formation in expanding steam. The present study is conducted to investigate the nucleation process occurring during the flow through convergent-divergent nozzles. A kinetic theory approach for determining the rate of droplet growth has been developed successfully. The governing flow equations are developed for the free and continuous flow regimes. A numerical scheme is introduced to solve the resulting equations. To validate the predictions, an experiment is carried out to measure the droplet size and thermodynamic properties of steam expanding through a convergent-divergent nozzle. It is found that the amount of supersaturation is limited with pressure increase and the limiting degrees of supercooling and the resulting droplet sizes depend on the Wilson pressure and the expansion rate. The theoretical predictions are in good agreement with the experimental measurements and the process of nucleation in high pressure steam can, possibly, be described satisfactorily by the model developed in the present study. (C) 1997 Elsevier Science Ltd.