The mass transfer for membrane extraction through a flat-plate module has been studied under concurrent-flow, countercurrent-flow and cross-flow operations. It was found that the mass transfer of solvent extraction in microporous-membrane mass exchangers can be analogous to heat transfer in heat exchangers, and also somewhat analogous to mass transfer of membrane dialysis except that in membrane extraction, both phases are immiscible. The correction factors for both concurrent-flow and countercurrent-flow operations were found to be the same and equal to unity. For cross-flow operation, the correction factor is a function of the fluid concentration and distribution coefficient, and is shown graphically.