A study on membrane dialysis in parallel-plate rectangular mass exchangers was carried out under cocurrent-flow and countercurrent-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. Theoretical predictions confirmed with the experimental results. The performance in a parallel-plate module arranged, by inserting a vertically impermeable barrier in the flow channel, to create a device of double passes in the retentate phase, was also investigated. Considerable improvement of mass-transfer rate in a parallel-plate module of fixed dimensions is obtainable if double passes, instead of single pass, are arranged in the phase (retentate phase or dialysate phase) where the mass transfer is dominated by its external film and not by the membrane resistance. It was also found that the separation efficiencies are the same for the operations of cocurrent-and-countercurrent flow and countercurrent-and-cocurrent flow, in a double-pass device.