A continuous flow colloidal affinity magnetic separation device is used for the removal of As(V) and Cr(VI) from aqueous solutions. Langmuir isotherms fit the adsorption behavior of the individual ions on Orica MIEX (R) ion exchange particles. In a mixture of equal weight percent As(V) and Cr(VI), the adsorption of As(V) begins only above a critical cut-off concentration, implying preferential adsorption of the higher valence ion at the available sites. Cr(VI) is removed selectively from the mixture in the continuous flow device, consistent with the presence of a higher concentration of the higher valence ion in the proximity of a charged (anion-exchange) surface.