We investigated the possibility of purifying oil-field produced water using. a bentonite clay membrane. Experiments were performed in a bench-scale cell which incorporates a piston to compact the clay membrane. We diluted a produced water sample with an original total dissolved solids (TDS) of 196,250 mg l(-1) obtained from a facility near Loco Hill, New Mexico, operated by an independent and tested the separation efficiency of the bentonite membrane for each of four dilutions. The results indicate that membrane efficiency for inorganic solutes decreases with increasing solute concentration and with increasing TDS. The rejection of SO42- was greater than Cl-. This may be because the SO42- concentration was much lower than the Cl- concentration in the waters tested. The cation rejection sequence varied with solute concentration and TDS. The solute rejection sequence for multi-component solutions is difficult to predict for synthetic membranes; it may not be simple for clay membranes either. The permeate flows in our experiments were 4.1-5.4% of the total flow. Both the flux through the membrane (approximately 2.5 x 10(-3) m(3) m(-2) per day) and the inorganic solute rejections were low. This suggests that clay membranes, as tested, are not suitable for purification of high TDS waters. (C) 2003 Elsevier Science B.V. All rights reserved.