The results of 11 experiments using compacted bentonite membranes in a cross-flow experimental cell equipped with a piston to maintain clay membrane compaction are reported. Due to dispersion in the porous frit, solute concentration buildup adjacent to the membrane was not a problem at the flow rates used in these experiments (6 to 126 mL/hr). The solute rejection efficiency of the bentonite membrane decreased with increasing solution concentration. The rejection efficiency for the 0.5-mm thick membrane ranged from 68% of Cl - for 100-mM (3545 ppm) NaCl solution to 13% of Cl- for 2300-mM (81,542 ppm) NaCl solution. The membrane exhibited stable solute rejection.