Cation exchange in styrene sulfonate gels of thickness less than 100 nm is described. The measurement, involving a radiochemical displacement proximity assay with Ca-45(2+) as a convenient reference ion, permits rapid evaluation of the kinetics and equilibrium distribution of ions. Because the films are so thin, the limiting mechanism for exchange is mass transport through a thin layer of stagnant liquid adjacent to the exchanger, as opposed to mass transport through the exchanger itself, as found with classical resin-based exchanger materials. Thus, equilibrium is quickly established with all cations, including large, hydrophobic ions, which diffuse very slowly through exchanger. Anomalous behavior in this system of well-defined geometry is readily apparent. The technique has sufficient sensitivity such that the thickness of the exchanger can be submonolayer. As an example of the potential for following, in situ, ion exchange at surfaces, a cationic poly-electrolyte is used as solution-phase displacer. The charged segments on the polymer compete with Ca-45(2+) for surface anionic sites. Subtle differences between the adsorption behavior of narrow- and wide-molecular weight distribution polyelectrolyte are revealed.