Hollow fiber membrane contactors offer several advantages over dispersed phase contactors for extraction of aqueous feeds, including higher inter-facial area, absence of emulsions, no flooding at high flow rates, and no unloading at low ones. The use of these contactors with extraction solvents near or above their critical points is of particular interest, because such fluids provide a favorable distribution of many solutes, high mass transfer rates, and easy recovery of extracted solutes. In this paper, we describe a theoretical study of acetone extraction from aqueous solution into supercritical carbon dioxide. with tube side CO2 flow either with or opposed to the force of gravity. Buoyancy-induced flow in the CO2 phase was important for large (1.8 mm inside diameter) but not small (0.6 mm i.d.) fibers, consistent with our expectation that such flow is more difficult to achieve when the characteristic length is small. The importance of buoyancy-induced flow decreased with increasing imposed fluid velocity, as forced convection masked the effects of free convection. For the range of conditions studied, the mass transfer coefficient obtained with flow in the direction of gravity was as much as 33% higher than with flow opposing gravity. A study comparing the performance of traditional mass transfer equipment and hollow fiber membrane contactors is also described. Ethanol and isopropanol extractions performed by others using spray, sieve tray or packed columns were run on a simulated membrane contactor, and the resulting values for the height equivalent to a theoretical stage (HETS) were compared to the reported values for the conventional contactors. In most instances the simulated membrane contactor offered a significantly (in some cases, substantially) lower HETS than the corresponding traditional column, indicating that the membrane contactor was more efficient. (c) 2005 Elsevier B.V. All rights reserved.