In order to advance the understanding of membrane transfer processes and their optimisation, models are required that can address all of the important physical processes occurring in membrane systems. This paper describes an extension of our previously validated computational fluid dynamics (CFD) model of a pressure-driven system to include a mechanistic model for permeate flux and buoyancy effects. As an example application, the effect of buoyancy in reverse osmosis of salt-water separation in a flat sheet system is examined. We show that for the salt-water system, buoyancy effects are important only for low flow rates that allow a significant increase in the salt concentration at the membrane surface and when the flow direction is aligned with the direction of gravity (i.e. in a vertical channel). No buoyancy effects are predicted for this system when the flow is oriented normal to the direction of gravity (i.e. for a horizontal channel). (C) 2004 Elsevier B.V. All rights reserved.