The methodology necessary to design a solvent extraction plant for isotope separation by chemical exchange is outlined. This process involves the calculation of the number of stages used in banks of mixer settlers used in such a plant. The feed is introduced at both ends of the plant, and the product is withdrawn at one end and the tailings at another. The resulting analysis, illustrated for the case of uranium enrichment, produces a fairly simple algorithm, and then more complicated configurations are examined. The method of design can be used for other systems where the isotope separation factors are more favorable.