This work reports lactate extraction with Aliquat 336, an ion-exchange carrier, using membrane contactors with hydrophobic microporous membranes.The overall mass-transfer coefficients were evaluated by either assuming a constant distribution coefficient or using the equilibrium equation. The mass-transfer coefficients calculated using the equilibrium relationship between C * f and C 0 , proved to be more rigorous, since the variation of the distribution coefficient with solute concentration is accounted for throughout the extraction process.Assuming the resistance in series model, the membrane resistance was identified as the limiting step on the mass-transfer process. In order to increase the membrane mass-transfer coefficient, it is necessary to increase the diffusion coefficient of the lactate-amine complex; this can be achieved by reducing the viscosity of the organic phase either through a temperature increase or reduction of the carrier concentration.Using a plate-and-frame module with a well-defined hydrodynamic characterization of both phases, the mass-transfer coefficients can be rigorously evaluated; these were compared with the values obtained with hollow-fibre modules.Simultaneous extraction and stripping of lactate was accomplished using two hollow-fibre modules in series. This configuration assures that saturation of the carrier does not occur, as it is permanently regenerated; furthermore, the carrier concentration can be reduced while still maintaining the mass-transfer rate, thus decreasing the associated operating costs of the process.