Sediments native to the US Department of Energy’s Savannah River Site in Aiken, South Carolina are ineffective at binding 137Cs, allowing it to remain available for biological uptake. Unlike the native sediments, illite has inherent characteristics that increase its propensity to sorb 137Cs in a nearly irreversible manner. The objectives of this study were to determine if the addition of illite to 137Cs-contaminated, native sediments would effectively reduce plant uptake of 137Cs, and to establish the illite concentration most effective in achieving that result. Two plant species, corn and soybean, were grown in native sediments amended with illite at concentrations ranging from 0 to 5%. The illite amendment effectively decreased plant uptake of 137Cs, as concentration ratios (CR; Csplant/Cssoil) for both plants decreased with increasing illite concentration. The 5%-illite treatment induced corn CRs to decrease by 29% and soybean CRs to decrease by 42%. The greatest incremental benefit was observed with an illite amendment of approximately 0.5%.