The performance of organic solvent nanofiltration membranes was studied in a pilot plant apparatus using a spiral-wound (STARMEM (TM) 122(1)) membrane element over extended periods, with 0-20 wt% solutions of tetraoctylammonium bromide (TOABr) in toluene. It was found that membrane transport parameters determined from flat-sheet membrane tests can be used to accurately predict spiral-wound element performance. A simple model considering uniform pressure/concentration conditions on permeate and retentate sides of the membrane element described experimental flux and rejection trends well for the system studied. The simple model gave similar results to a more complex model that allowed for spatial velocity, concentration and pressure gradients. However, when using a number of elements in series the simple model postulate of uniform conditions will be less accurate, and a complex model which includes spatial gradients, or a simple model that uses an average pressure obtained from experimental pressure drop values, will be required to give accurate predictions. (C) 2009 Elsevier B.V. All rights reserved.