A greater understanding of the factors affecting arsenic run lengths using ion exchange resins was achieved by performing an evaluation of the relative influence of anion-resin affinity and resin capacity. Separation factors were determined for nitrate, arsenate, and sulfate in both low- and high-ionic strength solutions using binary isotherms for five resins. An equilibrium multicomponent chromatography theory (EMCT) model was used to predict anion run lengths using experimentally determined separation factors. Of the common anions in groundwaters, only sulfate was found to have a significant influence over arsenic run lengths. Further, resin capacity was a much stronger determinant of arsenic run lengths than sulfate-resin affinity. This study also verified that selectivity reversal occurs during regeneration by determining anion-resin separation factors at high ionic strength, which were able to correctly predict low arsenic and sulfate leakage in the product water after spent brine reuse. Includes 7 references, tables, figures.