The ion-exchange separation theory of Mayer and Tompkins is extended to the case of multiple eluent ions. The selectivity coefficients for carbonate and hydroxide, each relative to bicarbonate, were estimated from retention data for bromide and sulfate. These results were used to interpret the chromatographic retention data for phosphate and arsenate as a function of pH in carbonate-bicarbonate eluents. Despite large relative errors in the empirical constants, the qualitative behavior of the polybasic anions was accounted for by the predominant effect of the triply charged ions on the retention time, even at low relative concentrations.