The effect of external recycle on the performance of dialysis in double-pass rectangular membrane modules was investigated. Theoretical analysis of mass transfer in parallel-flow dialyzers is analogous to heat transfer in parallel-flow heat exchangers. In contrast to a single-pass device without recycle, improvement in mass transfer of about 30% is achievable if parallel-flow dialysis is operated in a double-pass device of same size with external recycle which provides the increase of fluid velocity, resulting in reduction of mass-transfer resistance. (C) 2009 Elsevier Ltd. All rights reserved.