A study on mass transfer in countercurrent cross-flow rectangular dialyzers with external recycle was investigated. Theoretical analysis of mass transfer in cross-flow dialyzers is analogous to heat transfer in cross-flow heat exchangers. In contrast to a single-pass cross-flow device without recycle, improvement in mass transfer of about 30% is achievable if a double-pass device of the same size with external recycle is employed, which provides the increase of fluid velocity, resulting in increase of mass-transfer coefficient. Higher effectiveness may be expected when dialysis is operated for the system in which the mass-transfer resistances in the liquid films are comparable with that in the membrane.