Abstract: Sorption parameters are important components of models used to predict mass transport through dense or compacted earthen materials. These parameters are, however, generally determined in batch tests with loose, unconsolidated materials. Here we directly measure, using a specially designed cell, the extent of Cs+ sorption on bentonite compacted to a series of densities ranging from 0.50 to 1.50 Mg/m3, and compare the results with those obtained from batch tests with loose bentonite. The clay was saturated with a Na-Ca-Cl-dominated solution with an effective ionic strength of 220 mol/m3. The sorption data were expressed as distribution coefficients, Kd. Over the clay density range examined, Kd values for Cs+ with compacted clay are about one-half to one-third the value of those with loose clay. The lower sorption on compacted clay is attributed to small and occluded pores that Cs+ cannot enter; thus it cannot access the entire volume, or all the sorption sites, of compacted clay. The data suggest that reasonable estimates of Kd with compacted clay can be obtained by scaling down the Kd values measured on loose clay by a factor na/n, where na is the accessible porosity and n the total porosity of compacted clay.