Kinetics of membrane-based extraction can be controlled by the boundary layers and/or by the membrane transfer resistance, depending on the flow configuration and the nature of the membrane. While the mass transfer coefficients on a tube side of hollow-fiber contactors are well defined, there is a need to establish a suitable correlation to predict the coefficients on a shell side. Using several commercial hollow-fiber modules the extraction of ethanol from aqueous solution into n-octanol has been investigated experimentally. The effect of liquid flow rates, membrane type and packing density of modules on the extraction kinetics has been studied. Experimental results have been correlated by a new dimensionless equation for the shell side mass transfer, valid for low Reynolds numbers and high packing densities.