A zirconia-pillared and an alumina-pillared montmorillonite clay were found to exhibit high K(d)s for trace levels of cesium (2.5 ppb) in the presence of high concentrations of Ca2+, Mg2+, and Na+ ions with distribution coefficients (K(d)s) for Cs-137 exceeding 1 x 10(5) mL/g over a wide range of competing ion concentrations. An excess of K+ ions, however, caused a dramatic reduction in the Ca K(d)s for both of the materials. These results were in contrast to the parent montmorillonite which exhibited far lower K(d)s for Cs-137, particularly in the presence of high concentrations of Mg2+ and Ca2+. Batch tests in simulated groundwaters showed that both pillared clays efficiently removed Cs-137 from solution, with the best Kds being approximately 8.5 x 105 mL/g in a potassium-free groundwater. The materials performed less well in a groundwater with a high potassium concentration, but still gave K(d)s similar to commercially available zeolites. The rate of Cs-137 uptake was found to be rapid with both the alumina- and zirconia-pillared clays extracting > 90% of the Cs-137 from a 0.1 M NaNO3 solution within 5 minutes.