A study on solvent extraction in rectangular mass exchangers of microporous membrane has be carried out under cocurrent-flow, countercurrent-flow and cross-flow operations. It was found that mass transfer of solvent extraction in rectangular mass exchangers of microporous membrane can be analogous to heat transfer in rectangular heat exchangers, and also somewhat analogous to mass transfer of membrane dialysis except that in membrane extraction, both phases are immiscible. The separation efficiency increases with the flow rate of the raffinate phase, or with the flow rate of the extract phase, or with the ratio of the flow rate of the extract phase to that of the raffinate phase. Under comparable conditions most solute is extracted in countercurrent-flow arrangement, the least in cocurrent flow. However, the separation efficiencies of the three flow-type devices approach the unity when the flow rate in the raffinate phase or in the extract phase becomes very large.