The sorption of Cs + was investigated over a large concentration range (10 - 9 -10 - 2 mol/L) on subsurface sediments from a United States nuclear materials site (Hanford) where high-level nuclear wastes (HLW) have been accidentally released to the vadose zone. The sediment sorbs large amounts of radiocesium, but expedited migration has been observed when HLW (a NaNO 3 brine) is the carrier. Cs + sorption was measured on homoionic sediments (Na + , K + , Ca 2 + ) with electrolyte concentrations ranging from 0.01 to 1.0 mol/L. In Na + electrolyte, concentrations were extended to near saturation with NaNO 3 ( s ) (7.0 mol/L). The sediment contained nonexpansible (biotite, muscovite) and expansible (vermiculite, smectite) phyllosilicates. The sorption data were interpreted according to the frayed edge-planar site conceptual model. A four-parameter, two-site (high- and low-affinity) numeric ion exchange model was effective in describing the sorption data. The high-affinity sites were ascribed to wedge zones on the micas where particle edges have partially expanded due to the removal of interlayer cations during weathering, and the low-affinity ones to planar sites on the expansible clays. The electrolyte cations competed with Cs + for both high- and low-affinity sites according to the trend K + >> Na + =< Ca 2 + . At high salt concentration, Cs + adsorption occurred only on high-affinity sites. Na + was an effective competitor for the high-affinity sites at high salt concentrations. In select experiments, silver-thiourea (AgTU) was used as a blocking agent to further isolate and characterize the high-affinity sites, but the method was found to be problematic. Mica particles were handpicked from the sediment, contacted with Cs + ( a q ) , and analyzed by electron microprobe to identify phases and features important to Cs + sorption. The microprobe study implied that biotite was the primary contributor of high-affinity sites because of its weathered periphery. The poly-phase sediment exhibited close similarity in ion selectivity to illite, which has been well studied, although its proportion of high-affinity sites relative to the cation exchange capacity (CEC) was lower than that of illite. Important insights are provided on how Na + in HLW and indigenous K + displaced from the sediments may act to expedite the migration of strongly sorbing Cs + in subsurface environments.