A study on membrane dialysis in rectangular mass exchangers has been carried out under cocurrent-flow, countercurrent-flow and cross-flow operations. It was found that mass transfer of dialysis in rectangular mass exchangers of microporous membrane can be analogous to heat transfer in rectangular heat exchangers. The separation efficiency increases with the flow rate of the retentate phase, or with the flow rate of dialysate phase, or with the ratio of the flow rate of the dialysate phase to that of the retentate phase. Under comparable conditions most solute is dialyzed in countercurrent-flow arrangement, the least in concurrent flow. However, the separation efficiencies of three flow-type devices approach the unity when both the flow rates in the retentate phase and the dialysate phase become very large.