Recovery of sodium gluconate from aqueous solutions was studied batchwise in a pilot plant equipped with spiral-wound cellulose acetate or thin-film composite reverse osmosis membranes. The thin-film composite and cellulose acetate membranes had intrinsic membrane resistances (R-m) of (3.8 +/- 0.1) x 10(14) and (2.42 +/- 0.05) x 10(14) m(-1) and water fluxes of 60 +/- 2 and 91 +/- 2 dm(3) m(-2) h(-1) at a feed input absolute pressure of 41 bar and 40 degrees C, respectively. Osmotic pressure was found to be the major resistance to permeation. A model based on the resistance concept was able to predict solvent permeation flux within 5-11% for both membranes. (C) 2000 Elsevier Science Ltd. All rights reserved.